Can i give my 3 month old baby pear juice?

Not yet: Not yet unless your doctor recommended it as a treatment for hard stools (constipation). Juice may be started at 4 months of age but limit to max of 4 ounces. In my opinion water will always be better than juice. Specially since juice is "flavored sugar water". It does not provide any essential nutrition and abuse may lead to obesity.

Not advised: Unless your doctor is treating constipation. For the first 6 month of age your baby should only be on breast milk or formula. You generally don't want to give any juices at all (especially not the first 2 years of life) for many reasons: it "corrupts" those taste buds and then that's all they want to drink, most are way too processed, extra sugar added, empty calories, preservatives...