Technically you can do that but you have to calculate the power dissipation of the dropping resistor:
the formula is as follows:
Vin-V(load)/I(Load) ===>( 9V-3V)/I (Load) (we don't know that because you haven't given any specs for your 3V motor.
Can you post a link to the website that sells it ? (can you ask anyone who knows ? (like maybe your professor ?)
Power
example:
Let I motor = 300mA
P= I x E = 0.3A x 3V = 0.9 Watts ( 1W or greater dropping resistor ok)
Let I motor = 800mA
P = I * E = 0.8A * 3V = 2.4 Watts (need 3W resistor)
I suppose you don't want to hear that your 9V battery will either not work or only last a minute or two.