But would the Arduino have enough power to control just one 3V motor?
You can only safely get less than 40mA from an arduino pin so unless you motor runs less that that no you can't power it directly.
There are many ways to control a motor without having to buy a shield. Have a read of this:-
http://www.thebox.myzen.co.uk/Workshop/Motors_1.html
and the other motors pages on my site.