3 month baby can't finish one breast even if hungry..does it effect her health or growth? I feed her every 2-3 hours gap...

I would suggest feeding in demand basis rather than looking at watch and waiting for 2-3 hours. Is your baby gaining weight fine? Is she peeing at least 6-7 times in 24 hours? Is she sleeping fine? If answer to all the questions is 'YES' ,then don't worry. Your milk supply must be very good!