Masterfurry is right, the card should point out the 3-turn span, and when played it should have the traditional 3...0 counter. I also like OldTrees' idea about making it a flat +1/-1, instead of increasing each turn. This makes it easier to keep track of, reduces the potential abuses, and makes it more a true brother to Nightfall. Nightfall is specific-element and can't cripple your opponent, while this will drain his attack power, but is temporary.
Heaven forbid we should presume to design your card for you, but if you go with flat rate instead of increasing each turn, I feel like +2/-2 is more in line with taking up a space in someone's deck. That will provide a much more workable stability buff to friendlies, and the stall factor for your opponent won't be as trivial, and more worthy of permanent control.