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Once its ability resolves, removing one of the creatures with different powers won't allow a player to block Harvesttide Sentry with creatures with power 2 or less.
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.
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.
Harvesttide Sentry's triggered ability checks your creatures' powers both when it triggers and as it tries to resolve. If you somehow don't have enough creatures with different powers as it tries to resolve, nothing happens and Harvesttide Sentry can be blocked as normal.
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.
The blocking restriction is applied only when blockers are declared. After the coven ability resolves, if a player blocks Harvesttide Sentry with a creature with power greater than 2, reducing that creature's power won't make Harvesttide Sentry unblocked.