Hi,
Depending on aircraft weight and apron slope, on the TriStar, we'd start 2 or sometimes all 3, then during the prolonged taxi queues (JFK especially), we'd taxi on two engines (and shut the 3rd one down if started).
We used to get lousy mpg with 3 idling engines and the parking brake set.
But that was in the early 80s, now we are "catching up" again and doing it on A319 - 20s.