12 weeks is very young. Lots of puppy antics to come!
Chewing the baseboards is likely because he is teething, has a need to chew something. Give him something appropriate to chew on. Finding a safe chew can be a challenge, especially for aggressive chewers, so be careful.
I used Bitter Yuck spray when Max tried chewing on some furniture. It works for some dogs, but not all.
For the piddle pad, limit his freedom to a smallish area with the pad close by, especially when you cannot supervise. Be sure to praise and reward when he does it right. Clean up the accident spots with an enzyme cleaner to prevent him from wanting to return to those spots.