Can I combine the power supply?
If you are sure of the motor voltage and currents and all the motors
may need to be on at the same time you really want two voltages.

Using a single a 7V supply you would need to drop
4V to power a 3V motor. 4V * 1.17A * 6 motors = 28W. That is
a lot of power to dissipate. 

You can get dual output wall warts. Get one with a 7-10V output
and a 3-5V high current output. If size is not a factor you
could get a PC power supply. Use the 3.3V for the motors
and use the 12V for the Arduino with an regulator to drop
the voltage to 7V or so.

You could also get a single 3.3V supply and use a boost
converter to generate 7V. It would be a small boost converter
since the Arduino wouldn't need much power. These are
easy to build. Something like a MAX34063 or LT1302 would work.

I would do the boost converter solution.

One more note -- I would not use Darlingtons to drive the motors. Darlingtons
have a very high saturation voltage and you are going to
dissipate a lot of power. A better choice would be a MOSFET.
The IR IRLB8748PBF is a 40A MOSFET with an Rds(on) of 7mOhms (at logic levels).
At 1A you will dissipate around 7mW compared to over 1W in
the Darlington. They are about 90cents from Digikey.