Tremolo

Start slow; in the beginning it doesn't matter whether you group the pulses in threes or fours. What matters is that they go at a consistent rate. And the best way to develop that is to build up from a slow rate of pulsation.

Your neverous system needs to get familiar with the actions. If you try to go too fast too soon, it may skitter off in an unknown direction that happens to be the path of least resistance - which tends to be inconsistent.

Try this: set the metronome to 60 beats per minute. Pulse once per beat for several seconds on sustained notes, both blow and draw.

At first, you may have to consciously initiate each pulse. But as it become more familiar, you can just turn it on and let it go.

But before you get to that stage, try pulsing twice per beat at 60. Then build up your speeed from there. Later you can decide which division of the beat in your pulsation best suits any particular song. Straight 8th songs such as rumbas and two-beats will benefit from dividing the beat in 2 or 4, while shuffles will benefit from dividing it in 3.