My baby who's a yr old and 8months haven't pooped for 3days already. She's breastfed, but I'm thinking if it's because of the banana?

That is not a good sign, try to use Glycerin Suppositories. You can buy this at any drug store. This will help your baby to poop, ang sinasabi kasi ng pedia kapag breastfeed dapat daw hindi constipated si baby. Ask pa din your baby pedia