My baby had it when he was a little older, I think about 15 months, and he felt pretty crummy for a few days. He didn't want to eat anything but he was okay with nursing. The pediatrician recommended mixing Maalox and Benadril, which did seem to help -- the Maalox coats the mouth sores and makes it less painful, although I don't know that you could give it to a 3 month old.
If you do have it, the good news is that you're passing all your adult antibodies on to your baby every time you nurse, so that will help her get over it faster.
Jen, journalist, policy wonk, and formerly a proud single mama to my sweet little man Cyrus, born at home Dec. 2007
. Now married to my Incredibly Nice Guy
and new mama to baby Arthur.