Can I give my 3 month old baby pear juice?

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...
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.