I have been in a relationship with my partner now for nearly 3 years, and we are engaged and have a baby at first things were great between us but now i feel like he doesnt want to be with me.

Having a baby can sometimes have that effect on some guys, if he feels you have pushed his nose out to concentrate on the baby too much.  In reality though a guy should be overjoyed about having a baby and want to look after the baby and look after you.  You haven't given me much information to go on here, so I would have to speculate about your circumstances.  Some guys are just interested in sex and women and only sometimes have a vague interest in starting a family.  If you have a child by accident or too soon in the relationship they lose interest.  This is the wrong type of guy to choose but lots of women make the mistake of choosing this type of jack the lad bloke.  Indeed these guys may eventually look at wanting a family, it may just have been too soon for this guy.

I catch him looking at other girls. 

This really isn't that significant.  A lot of women are always saying to me that their partner is looking at other girls, by the fact that a lot of women say it, should just rid you of worry because it's a common thing.  Guys find women attractive, that doesn't change when your in a relationship, what should change is that they should see you as a better option and be faithful to you.  He may dream about being with another woman, in reality these other women maybe too attractive or not that interested in him and you have a far greater advantage to these other women because you spend a lot more time with him and he knows you.  Yes, he might look at other women, but I'm sure you look at other men and think they look attractive and I bet you don't follow through by showing interest in those guys.

He never touches me anymore.

This is a negative situation and can be the result of several things.  When women become pregnant, some men feel as though they can't have sex with their women.  Some women's bodies also become less attractive both during and after pregnancy.  These thoughts sometimes stay with a guy. 

The other thing that relationships experience is they go through stages and you are at one of the later stages.  Described as stage 5 in the nonverbal dictionary and here is an extract from stage 5: "After physically bonding in love, there is less need to renegotiate the closeness achieved in previous courting phases. Loving couples thus emit fewer love signals. Since they take the distance between them comfortably for granted, they give off fewer "come-hither" cues" David B Givens .

Based on this statement you may think this is a relief and nothing to worry about but in your case he is not giving any touching signals and although giving less at this stage in a relationship should be satisfactory, giving none is not a good for a relationship.  Clearly it is not good for you because you have stated that you are unhappy that he doesn't touch you anymore.

Now to look at a way of changing that situation.  You need to re-invigorate your relationship, you need to try to move back to stage 1 of your relationship or at least pretend your back to stage 1.  That means you are trying to convince each other that you want to go out with each other.  You dress up sexy, you put on the makeup, you go out together, almost likely your starting off in a new relationship.  If executed in the right way, he should be thinking it's exciting it's like finding a new woman.

He sounds to me as if he has become bored with your relationship and basically the best way to deal with that is to gradually make the relationship more interesting to him again.  Yes, he is most likely taking your relationship for granted but it maybe that having a child and being together so long, you have given him the idea that you are taking the relationship for granted too, and are not looking for anything more out of the relationship from him. 

He wont sleep with me anymore. I think that he is having an affair.

Unfortunately, nowadays there are a lot of women that will have casual sex.  This has increased the number of women available to men and caused a breakdown in relationships and increase in divorce rates.  Men ultimately want sex and to have sex with as many women as possible is just a dream to men.  Women are now providing that feature to men, as they are indeed striving to have sex with a wide variety of men too.  For this reason women in serious relationships have a tough battle to hold onto their men, because it seems that most women have a preference towards men that have girlfriends or partners.  Almost like these guys are untouchable which makes the challenge even better to these women. 

If he is not sleeping with you anymore it could be as a result of several things.  The most likely as you indicate is he is having an affair, and getting that exciting sex from somewhere else.  In which case it's your challenge to get him to be having that exciting sex with you again, persuade him that sex can be exciting with you.  Sometimes it's not a bad thing if the woman gets a bit sexually aggressive or starts to touch him up at a time he least expects it.  You just need to catch him when he is not concentrating on something important.  For example you may go to the supermarket shopping.   When you are in the car, you may put your hand in his pocket and start playing with his loose change (well you know what I mean).  Little things like that, put some sexual excitement in his mind, with you involved in it.

You need to signal to him that you are sexual and you do want sex with him.  Sometimes it doesn't hurt for a bit of role reversal where the woman becomes the sexually aggressive one, as long as you don't go over the top and look aggressive, you need to try it gradually and in moderation.  I found a good 6 page article on another web site about male orgasms which you may find useful: http://www.redbookmag.com/289143

If he really has switched off with you, it might be more difficult than you think because if you do get more sexually aggressive then he may back off and say "what are you doing?" and try and push you away.  So as I indicate it is better to change gradually and once you made some of those changes then be more sexually aggressive.  From what you have said it's not going to be easy convincing him, because it sounds as though you have reached a fairly low point already. 

The final option for the reason he is being like this, is he is depressed about something, his job, his life and really has no appetite for sex or women anymore.  Whilst I say this, a relationship really should involve discussing these intimate things with your partner.  Men though tend to bottle things up when they are stressed out, because they see it as losing their masculine side if they show something is bothering him so much. The looking at the women thing on it's own didn't concern me, but the whole picture that you have given me from how the relationship is puts that looking at women thing into more perspective.

What shall i do?

First of all, don't accept defeat, you have got to be determined to win his interest again.  I want you to look at this posting and look at ways that you can change things for the better, NO dwelling on what is happening at the moment.  Secondly you need to GRADUALLY change what you are doing now, until you find a winning formula, not just change overnight because that will look false.  Whatever you are doing now, it's boring him, it's giving him no incentive to touch you, have sex with you or be with you.  You have got to try to change that.