He is married

I started a job 3 months ago the same time as a male colleague who is a few years older than me. Over time we have chatted and I have found myself attracted to him. I get from his body language that he likes me too. He gazes at me when he talks or he will come over to me and talk nonsense just so he can speak to me. The problem is he is married with 3 children.

I have to shut my feelings off for him and act as though they don't exist. He makes things awkward as I will walk in the office and I catch him staring at me or he will go in to the kitchen when I go. I don't think he is happy at home as he moans about his children and how much they cost and he sighs at how he has the children on his own at weekends as his wife works shifts. Today I got flustered as he came near me, I know I cannot involved but how can I deal with this knowing he fancies me too.

never get involved with anybody who is committed to another person (be it marriage, bf/gf, or other). the 3rd wheel always loses.
if they were going to leave that person for anybody - they would'n't be with them now woudl they?

next.

He wants a side piece be smart enough not to fall for the dudes sob stories -- most times they are lies to make whatever chick to feel more for them and get into bed.

He doesn't make anything awkward, you interpret it as awkward because you can't control your feelings of infatuation with him. You are infatuated with him. So what? He is married, that is a big problem, and his behavior does not so much suggest that he is equally into you, as it suggests he is friendly, and maybe even flirtatious in nature. However, that does not mean he would consider starting something with you and/or leaving his wife and children for you. Not only that, but have you considered the fact that even if he wasn't married, he may not want to date someone from work? I suggest you focus your attention on doing well at work, and not your crush on this co-worker of yours. There are plenty of men you can feel the same way about who aren't married.

He will obviously not leave the children or the wife (or he would have done so already)
if you are ok with being his sidechick then i dont see the problem
if you are not ok with being his sidechick then you could just take the flirting as a compliment.

You could start dati g and then talk about your dates with him. Talking about other guys, past or present should cool him down.