To get TREECKO!
Right after you defeat Ard, the bug maniac for the first time, go to the most northern point of the park. Slightly to the right, you will notice that a piece of the fence is missing. If you go right here there is a secret path going off to the right. You will be able to tell where it is as the trees look different there. The path leads to a hidden area where there is a girl. Make sure you do not have a full pack of pokemon. Fight her chansey and delcatty, and if you win she will give you a Level 1 Treeko
To get MUDKIP!
You can get Mudkip in the union cave I just accidentaly caught my Mudkip there Just Be Patient just walk around But dont go to the Water Part of union cave.
To get TORCHICK!
I havent seen any torchick yet !