best choice of a stepper motor to lift a 3kg load
Assuming your 3kg is not perfectly balanced, you're probably going to want a gearbox. I would recommend sticking with a regular size nema 17 stepper motor, and attached that either using a chain and sprocket, or for a cleaner look a timing belt and 2 pulleys.

Use a very large wheel on the rocker, and a very small wheel on the motor to create a large gear reduction.

A worm gearbox would be optimal, because it will prevent the rocker from tilting back on its own once power is removed from the motor. However, I didn't see many of those available in the right size.