In fact, I did cut the wall behind the tub as well as the wall behind my kitchen faucet. There is no manifold behind any of them. (I tried to attach the photos but for some reason the upload failed.)
Behind the tub, there is only one 1/2" pipe coming up to the tub. Behind the kitchen faucet, there are only two 1/2" pipes (one to kitchen faucet and one to outside faucet). Since I still couldn't figure out where the kitchen faucet and outside faucet come from, I decided to test pipe #1丶2丶3 again. Then I found something bizarre.
The first time when I end capped pipe #3 (the pipe from water meter), there was no water (none) in the whole house. But the second time when I end capped pipe#3, for some reasons there was water coming out from kitchen faucet and outside faucet. Arrrrrrrgh, that explained all. It took me 3 days trying to figure out where is another manifold. =(
So sorry for the confusion.