Good shield. Too bad it only lasts for 3 turns.
While I agree with you from the "if you're the one using it" point of view, if it lasted too many more turns it would be exceedingly broken. It lasts 3 turns, meaning if you have 6 (and get them all out) that's 18 more or less untouchable turns. If it was even buffed to 5 turns (30 turns with 6) it would have incredible effects on the game, probably not good ones either.
Creating a control lock is fine and good, it's a standard deck type in any good CCG. But if you make the control lock too good/unstoppable then it throws the entire metagame into "this deck" and "counter to this deck".