If I have trumpets on my spearman and do a move 3 modified by a -2 hard turn there is no 3 hard turn.

What happens next?

The rulebook addresses this.

If there is no template matching the speed and shape of the maneuver, you use the next closest speed of template, so the fastest hard turn is always a 2.

Just because I ran into this too and had a hard time finding it, not to be too much of a rules lawyer:

It's 56.2 last point.