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.