3 phase computer chip
Most chips don't run on AC, rather they use a DC power source. Computers have built-in AC to DC converters which can take the AC power from the wall and change it to the required DC power the computer needs. It's also entirely possible to run a computer using other power sources like batteries, solar cells, etc.

The oscillators inside of computers are designed to take a DC power source preferably with as little ripple as possible and change this to the high frequency clock used by the chip. If you're interested, do a search for "Pierce Oscillator" and "Phase-locked loop".

If you multiply the frequency of most 1-phase AC power lines by 3 you get a frequency of ~150-180 Hz, a far cry from the GHz range modern CPU's run at.

Consider this, though:

There are people who routinely overclock their CPU's to extreme limits (in excess of 5GHz). This means it's possible for someone to take the same CPU and theoretically pump a much higher clock through them. There are also oscillators available which can generate oscillations in the terahertz range. Why aren't these used in your computer? Because most chips will not function correctly at these high speeds under "normal" conditions. Case and point? These extremely overclocked CPU's use liquid nitrogen or other specialized cooling systems, and even then they can't be overclocked too much higher to the 10's of GHz range.

Lastly, CPU frequency isn't the only factor in overall computing power. In the early 2000's Intel was selling Pentium 4 chips which achieved ~3.8 GHz CPU clocks.

My laptop also has an Intel chip, an i5-m430. Even though this runs off of a 2.3 GHz CPU clock, I would say my laptop has much more computing power than the Pentium 4 computers.

Response to Rocketmagnet's comment:

So what would a 3-phase oscillator computer look like? Well, first you'd have to figure out how to hook up the oscillator to the CPU. I would guess you can hook up several single-phase chips between two wires of the oscillator, looking something like this (from Wikipedia):



In essence you have a triple-core chip which all have out of phase clocks. However, I fail to see how this is beneficial over a triple-core chip all sourced off of the same clock. If anything I would think synchronized clocks would be a beneficial feature when it comes time for inter-core communication.