Most of those boards have individual switches for each channel which set for 25, 50, 75 and 100% of the max setting. Depending on the resistors on the board, the max setting seems to be either 1.5 (0.33ohm resistors) or 2.3A (0.22ohm resistors) from what I have seen/read, very few of these boards are set up for 3A as stated in their adverts (nor do they have the heatsinks capable of that current output). Depending on the board that will give you 1.125A (75% of 1.5) or 1.15A (50% of 2.3) as the nearest to 1.25A. You could of course change the resistors on the specific channel to set 1.25A as the 100% value (0.4ohms I think) but that may affect any warranty you thought you had...
ALso if you haven't read the other threads regarding these boards be aware of the maximum voltage of 30v (absolute maximum) and the need to use low inductance motors if you want the board to survive past your first power down! Ignore the vendors specs, they are meaningless.