You're strong enough at the start when your drive leg (under load) can extend at the same time as the knee comes forward (no load). If the knee is well forward before the rear leg has much extension, you're too weak. Triple extension is just a way of describing a position where the body is extended through the hip, knee, and ankle. If you have enough strength for the start, you have enough strength for triple extension, as long as you are relaxed and flexible enough If you concentrate on the arms at the start and forget the legs, the legs will take care of themselves.
1: A mechanically efficient start can be defined by speed through the acceleration period.
2: the efficiency/economy of the start will be reflected, in part, by the athletes ability to finish the race.
Most start problems are correctable, regardless of strength levels, by adjusting the start position to reflect current strength levels rather than to establish one final position and wait for strength to catch up. This is not only possible, but necessary, as the start affects the smooth execution of the whole race. When strength is lacking the COM should be raised as much as possible in the set position by insuring that the arms drop straight down from the shoulders to the line (in both planes), that the hips are as high as possible while still maintaining enough bend at the back leg to ensure force on the back block, and a position close enough to the line that the athlete will automatically raise his COM as fast as is necessary. The start can be re-adjusted in lock-step with strength improvements (the needed changes, while effective, won't be huge).