How much time do we need to pump each breast to maintain good milk supply for exclusive pumping mothers and at how often we need to pump for 3 month old baby?

80% of breastmilk is extracted during natural feeding at 5 min/side/90% at 10 min. If you have a good automatic pump you can likely maintain a similar efficiency and output if done every 3-4 hours. Manual expression rarely keeps up with demand, but you can try a similar schedule. Some have more luck than others. Having the kid occasionally feed naturally can help maintain supply.