Not one person at any decent level serves with thumb and forefinger only, in match play.
Very very few decent players serve using 2 fingers and a thumb.
Very few players serve using 3 fingers and a thumb.
You could serve standing on one foot too, but nobody else does it.
One finger serve is a practice technique to work on a loose freeswinging racket head...only.
It get's maybe 75% speed.