it can be done....
i survived a very intense one year grad program with two children and pg with my third. in the end i missed the gpa cutoff to continue on as a med student by 1 question...but the point is that my grades WOULD have been good enough if i had been a reg med student.
yes there are sacrifices...anything you do has sacrifices. yes i may not have gotten to spend as much time with my kids as i would have liked. but i did get to spend time with them....i did not miss a single practice or game. and my husband is NOT a sahd, he works and in fact is in the process of pursuing his phd himself...we are a duel student family. yes things sometimes get crazy but over the years we have learned how to deal with it.
the point is that if you really want a thrid child it is possible