agree with Sage "rubbish" (okay I just wanted to say rubbish, we don't say that here
). He fell for who he fell for and lied about it. He was cheating. That is not poly. Poly is built from a set of foundations. At least in my life anyway. One of which is honesty. The others are open communication, empathy/consideration of others I am involved with, integrity and respect. These threads might help http://www.polyamory.com/forum/showthread.php?t=2858 http://www.polyamory.com/forum/showthread.php?t=2755
To me, when someone cheats they are not acting out of any from a place that indicates they value the four things I listed above. I suggest he look long and hard here and elsewhere and get his acted together. You and the other woman should do the same thing I think.
This is totally salvagable and could work, but you all have a shit load of work to do, starting with the pain he has caused by being deceitful. That trust is extremely hard to get back... in fact I highly doubt that it ever comes back entirely. It lasts from one relationship to the next and tarnishes everything.... I speak from experience with this one. My own.
If I were in your situation I think I would be getting my own life together and planning on my own, in case he doesn't work out. It's you and baby now. I wouldn't trust him as far as I could throw him... I would not want to be starting a life time of parenting with a man who has lied to me for three years. Ya, he would be a good baby daddy, but I would expect nothing of the truth to come from his mouth from here on in and move on to someone who is willing to rise to the occasion and be a stand up man and dad. Speaking as a mum, you will benefit more and so will your baby.
I know this is hard to hear. Take it or leave it... its your choice, its just my opinion.