Anyone can get thrush and thrush is not HIV specific. Antibiotic use can cause thrush or anything that disrupts the good bacteria that is found in the mouth. The only way to have thrush diagnosed is by seeing your doctor or dentist.
Also as an addition to what these posters have said...
You wouldn't get oral thrush until years after acquiring HIV. It's not one of those things that shows up soon infection, but rather years down the line after significant immune system damage has been done.
I agree with verystressed07.
unless you're an IV drug user, on chemo or on antirejection drugs (?), your 3 month neg. is conclusive unless you've been exposed some time after that. You might want to ask your doctor about that. This is what Dr. H would say as well as Teak.