Maybe you need a variable speed motor. You will be able to get the right motor for your application if you can also specify the LOAD for your conveyor.
This link might be useful if loads are not quite large. https://makermotor.com/variable-speed/
On a lever, how far is the 3kg load from the motor's rotation axis? If you have this information, it will give you the torque required. Then, with torque and speed, you have access to power and you have enough data to select a motor. Are you looking for a single motor or to buy several? In which place are you located? What is your business case?