Problems with Ring Oscillator using CMOS inverters in Cadence
I just saw that you only have 2 capacitors in the circuit. 

If they are large (say over 1000x the input capacitance of the inverters), then this won't work. Basically, the phase shift of each inverter driving a capacitor needs to reach nearly 90 deg (if the 3rd inverter had no capacitive load, its phase shift will be small) so that the total can reach 180 degrees. 

If the capacitors are only on 2 inverters, by the time these reach nearly 90 degrees phase shift, the attenuation will be large (say 10x); the remaining inverter is unlikely to have a gain over 100x, and so the total loop gain will be &lt; 1, and it won't oscillate.

So -- put a 3rd capacitor and it'll oscillate at the point where each stage has 60 degrees of phase.