Think Smith Rowe being ever so slightly behind the other two in the rate at which he's selected is more to do with his talents being a little more generalist, subtle and ambiguous, reading the game, creating, scoring in specific situations etc.
Saka and Martinelli both have clear "weapons" in their currently assigned wide roles, such as the capacity to beat a man and finish reliably. Saka has really advanced physically as well. Those weapons are keeping Smith Rowe out of the first XI half the time, as well as giving those two the chance to develop fast.