Well, depending on how many months pg she is would determine how much she FEELS like doing.
Of course there are certain rides that are restricted (and those may be height restricted for her 3 yr old anyway) so she should talk to her doctor and determine what her doctor says is safe. I know that Space Mtn, Indy, Big Thunder, Matterhorn, Splash Mtn, Star Tours, not sure what else is restricted.
There are still a ton of things to do and lots of fun to be had!! She just needs to pace herself (as it may be quite warm) with breaks and to drink lots of water too.