The metronome everytime just follows the style that is selected at the time,even if you don't press the Start button.
When we turn on the keyboard the metronome is at 4/4 because the style at that time is the first preset style of the model.
E.g to my keyboard the first preset style is the Hard Rock 4/4 style with tempo 120.
An alternative to make the metronome play a 3/4 rhythm all we have to do is by selecting a selecting a 3/4 style and adjust the tempo.
Personally I find the sound of the metronome kind of annoying so if it is just for practice I prefer to play with the sound of the main A drums with the ACMP button in off position to keep the correct rhythm.