You have it slightly backwards. The Uno clock frequency does not change with the voltage on the supply rail; it depends only on the crystal oscillator frequency you are using.
BUT...at lower voltages, you cannot use higher frequencies (see Figure 29-1 in the ATmega328P datasheet for example). At 4.5V and higher you can run at a full 20 MHz, but at 2.7V you can only run at 10 MHz. If you try to run an ATmega328P at 16 MHz and 3.3V it probably just will not work (or not work reliably). My calculations say the maximum frequency at 3.3V is 13.3 MHz.
The Basic Motor Driver
: simple, inexpensive motor driver for 1 stepper motor or 2 DC motors