This reads like a drawback, so in order to feel like an upside mechanic it should say something like:
Triad — if you control exactly three permanents with triad, <bonus>
However, it's important to consider that it's not that easy to consistently have even 3 normal creatures in play at once, so it could be very harsh to require 3 triad permanents in play before getting any bonus. I do think there could be designs that assist achieving Triad.
There could be a Common card like:
Sacred Oasis
Divine Land
T: add 1 to your mana pool.
Triad — If you control exactly three Divine permanents, add one mana of any color to your mana pool instead.
However, I think the biggest difficulty with Triad is that it forces you to choose cards for your deck from only a small group of cards. It will be horrible in draft, because it would force you to go all-in, and rob you of any real choice-making once you start down that path.
Also, since Triad is so hard to achieve, the bonuses should be huge. But if 3 Triad cards were to each get those huge bonuses, it would be an extremely swingy mechanic.
I think Triad would work better as something like this:
Sanctifier Cleric 1W
1/3
When ~ enters the battlefield, put two sanctity counters on target permanent. (Permanents cost an additional 1 for opponents to target with spells and effects for each sanctity counter on it.)
Divine Owl 3WW
4/4
Flying
When ~ enters the battlefield, put two sanctity counters on Divine Owl. Then you may put two sanctity counters on up to one other target permanent.
Triad — If you control exactly three permanents with sanctity counters on them, those permanents are indestructible during your turns.
I got the idea of sanctity counters as a fix to the sanctify mechanic in the proposal by Jacob Munford.
The point of the cards above is this:
1) The counters shouldn't be useless on their own. (Triad could also work with +1/+1 counters instead of sanctity counters, or any counters, so that it would play well with cards from other blocks.)
2) The Triad cards should grant the counters to other permanents, so that you don't need to draft or construct an all-Triad deck. Not only is Triad easier to achieve, it would also mean that different cards can be inducted into Triad-hood each game, lending to variety of game play.
