Needed Servo Torque

[DonRotolo]

A torque of 125 oz-in means that you can push with 125 ounces of force at a distance of an inch radially from the center of the servo 'axle', or 1 ounce at 125 inches away. (Or 62.5 ounces at 2 inches, etc.)

3 pounds is 48 ounces, the math estimates that it must be about 2.5 inches away at most to 'lift' it. But that's lifting it against the force of gravity. You probably only need to Move that weight, and so you must find out exactly how much force is required to move it in the way you want. A fish scale is a good way to determine that experimentally, ask the Physics or Technology teacher to borrow a small spring scale to measure force.

Without any details of your mechanism - exact details - we can't even guess.

But weight, friction and distance are all your enemies. 125 oz-in is 'best case' - plan on 2/3 to 3/4 that for a factor of safety.