Please help me out with EV training - Pokemon Diamond?

Well here it goes. I recently hatched a Sneasel from an egg and decided to raise it's SPEED and ATTACK. I decided to raise it's SPEED first. Straight after it hatched I use 10 Carbos on Sneasel and equipped it with Power Ankel. Then I went straight over to Fishermen Andrew, the trainer with 6 Magikarps. Now for the past 30min I've been killing only Magikarps on that one trainer. Then when I level up my SPEED stat on raises by 3. This had been happening EVERYTIME I've leveled up. Even now my Sneasel is level 25 and still only getting a 3 stat bonus to SPEED. Just to remind you I have Power Ankel equipped. Now the question is, why am I always getting 3 stat bonus to speed everytime I level up.