A set v.three of a kind.

The terminology for having trips with two in your hand and one on the board is "set". For example (55) 9 5 2.

If you have trips with a pair on board you don't have a set, you do have three of a kind. For example (56) 9 5 5.

This isn't some artifical distinction based on terminology. Theres a real difference, the set is much, much stronger, and not just because it's hidden.

Say you have 55 with a flop of K 9 5

How many hands beat you?

Compare that to a hand of 54 with a flop of K 5 5

How many hands beat you now?