@gklee The first thing to keep in mind is that your puppy is a baby. 3 months old is still a really baby dog. Just like with a human baby, they take a lot of training and attention. Also like a baby, he needs a lot of naps. My lab is 11 months old now but when she was 3 months, I put her down for a nap every 2 hours or so. Even now, I make sure she spends 2 hours napping in her crate every afternoon. Getting enough sleep makes her a much happier dog.
Second, to help with the biting, he needs a lot of socializing. It's from other dogs the he'll learn how to chill out with the biting. Join a training class with puppies the same age. If you're near a Petco, they usually have a puppy playtime every weekend. Puppy playtime is overseen by a training professional to keep things from getting out of hand. I highly recommend it.
Third, exercise is important but, again, remember that he's just a baby. Puppies shouldn't walk more than 5 minutes per month of age. So 15 minutes for your little guy. Otherwise, tire him out with toys and training.
Fourth, leaving him unsupervised for any amount of time at this point is going to result in things you don't want happening, like killing plants. If he's awake and not in a crate or exercise pen, he needs to be supervised so he doesn't get into trouble. If he starts heading toward a plant, redirect him to something appropriate. Ditto with biting. Redirect him to an appropriate thing to chew on.
Finally, when it comes to potty training, at 3 months old your best bet is to take him out to potty every 2 hours. Get him outside to pee before he realizes he needs to go so he doesn't pee inside. I recommend starting bell training now, too. Get a set of bells to hang near the door. Every time you take him out to potty, ring the bells and say "outside." As he gets older, you can take him out less frequently and he'll learn to ring the bells when he needs to go out.
Good luck! Puppies are so cute but also so much work. You can do this.
