The card text of "Tenacious Z" says:
Special: During your turn you may play this card from your discard pile as an extra minion. You may only use the ability of one Tenacious Z each turn.
I'm not sure if that means "once for each card" or "one time each turn, no matter which card it is". So may I play 1 or 3 Tenacious Z from discard pile when I have 3 of them in the discard pile?