For three creatures to have different powers from one another, each of their powers needs to be different. A 1/1 creature, a 2/1 creature, and another 2/1 creature aren't three creatures with different powers, even though both 2/1 creatures have different power than the 1/1 creature.

A creature has different power from another if their powers are different numbers. For example, a 1/1 creature and a 2/1 creature have different powers.

The effects of Might of the Old Ways happen in the listed order, so the +2/+2 bonus happens before you check the powers of creatures you control. For example, if you control two 2/2 creatures and a 5/5 creatures, choosing one of the 2/2 creatures as the target will allow you to draw a card. Conversely, if you control a 1/1 creature, a 3/3 creature, and a 5/5 creature, targeting either of the two smaller creatures will not allow you to draw a card.

Many coven abilities, such as that of Dawnhart Wardens above, are triggered abilities with intervening if clauses. You must control three or more creatures with different powers at the time the ability triggers and at the time the ability tries to resolve. They do not, however, need to be the same set of creatures in both cases.