I'm replacing an emerson blower motor (single Speed) with a mars 3 speed motor. The emerson has 3 wires to the blower motor red, blue, and black. When I connect the mars motor with the sama color wires nothing happens

Make sure the motor you are replacing is the same voltage and horse power.

All new motors come with a wiring diagram on the side of the motor or in the box it came in. Use it since the old motor might be wired different.