>>5099831 (lb)
Negative. Main engines start at T-6.6 seconds.
The engine start sequence requires 3 and a half seconds or so for ignition, that's why the mains fire about 3 seconds before T-0, which is when the boosters fire. Ironically, actual liftoff occurs about 2 seconds after the boosters fire. Also, there are hold down bolts that must release before the shuttle lifts off.
That being said, let's focus on the fact that a countdown has started. We don't know where it's at, and there may be holds. But here we are.