Danny Deger wrote:
Why does the shuttle throttle to 3 Gs on ascent?

As the fuel is burned/jettisoned the engines have a smaller vehicle mass
to accelerate. So to maintain an acceptable acceleration level, you
throttle back engines.

Also, as you leave the atmopsphere, there is less and less air resistance,
so you also need less power to maintain the desired acceleration level.

And early in the ascent, they throttle back to ensure the shuttle's air
speed isn't too great as it travels through the still dense atmosphere
since it would create too much aerodynamic drag.