Here is a thread which discusses positions:

http://www.sexinfo101.com/forum/sexual_health_women/13206-sex_pregnancy....

Also do a search on this site. I recall Brandye discussing this topic before to another poster who had the same question. I just don't recall the name of the thread. Use the search option on the upper right, in the blue area.

In the third trimester, sex is still desired by many women but pressure on the tummy needs to be avoided. A position I like that would apply her is to lie on your back with your knees over your partner lying crosswise on his side. Insertion is easy and each partner has complete access to all critical parts of the partner or self.