There a minor bug in HexRoller right now.
If you select the "Place a 3rd group" bonus in a round and then undo it and finish the round with only 2 numbers as usual, the bonus is active at the beginning of the next round (it should only be after you have placed 2 numbers). If you "use" it at that moment, the game seems to continue normally but you only place 2 numbers, not 3. And your bonus seems to be considered used anyway.
So if the bonus is active too early as a result of the bug, make sure that you don't click on it before placing 2 numbers.