Me and my ex split up about 10months ago after a three year relationship he went in the army and said it was to hard for him to have a relationship.

I doubt that was the real reason why he split up.  Yes, it is difficult to have a relationship when you are away a lot of the time, but lots of other soldiers manage.  The real reason was most likely he either had what he thought was better option with another woman or he thought your relationship wasn't really worth it anymore. 

but i found out a month after we split he got with his sisters mate

That's probably the reason why he split with you because this other lady was tempting him.

but was still and still is saying he loves me

After a three year relationship with someone you are bound to be missing them and you are more than likely to still love them.  Whether he should really get back with you depends on how much he has missed you and whether he believes your relationship could be better second time around.

and her and that when they sleep together he sees my face

It just demonstrates that he is not over his relationship with you and is maybe having second thoughts about his relationship with this woman.  Being away in the army, he probably doesn't get to see this other woman that often either.  Maybe he likes the excitement of sleeping with her but really wants to be with you.  Maybe he has been tempted by this lady who is sexy and who he can have great sex with, but who he has no long term rapport with.  On the other hand with you he had a long term relationship and there was obviously some rapport there for you to last three years together. 

help i dont know what to do.

It's really up to you what you do.  If you still want to give him another chance, then I reckon you should get yourself noticed.  By that I mean where does he go in his spare time?  Does he go to a local pub, maybe you can go down to the pub, get his attention and see if he approaches you, keep doing it until he does.  From there you may find out he is not enjoying himself with this other lady and may regret finishing your relationship.  Whichever way, if you feel you can make a go of it second time round then you've got to at least try.  It will be a case of dipping your feet in the water at first and testing out whether he really wants to be back with you.

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