Is it feasible to install 3000 mAh battery to MMX A89 mobile?
According to this site, your Micromax A89 Ninja phone has a removable back plate with a replaceable battery.  It should be possible to replace the original 1400 mAh battery with a higher capacity one. However such battery will definitely be thicker in size, so you would most likely need to buy a different back plate to accommodate for the added thickness (see an example for Galaxy S4 here.)

I have not found any online merchant that sells extended batteries for Micromax A89, but you may have better luck with local cell phone dealerships in your part of the world.