What is the current requirement of the motor? If it's less than 60mA, you can probably get away with sinking the current.
Be aware that some digital pins can sink more current than they source. At 60mA, you will still be at 1.5V out (so watch your power dissipation!), but you won't get far at all trying to source that kind of current. 40mA is a generous maximum for sourcing. Therefore, you should connect to the negative terminal of your motor.
If you need to use multiple outputs together, be aware that the transistors in those outputs will likely have subtle differences, and not share the current very accurately. A <10 ohm resistor on each output should help them to share the current more evenly.
Try to split shared outputs between different ports on the microcontroller, because this will separate the heat on the chip. This has the drawback that it takes more time to set multiple ports, so try to avoid doing read/write/modify if you can help it.
Finally, as Joby noted, don't forget a diode.
Edit: I'm assuming this is a simple DC motor, and you want to operate it in one direction. If you have anything more complex, you should brave the cold and get yourself a proper driver.