Hi,
I am trying to design the same. Attached is a document that shows a differential ring oscillator.
Did you get a simple single ended three stage oscillator running?
Try that first to see if you can get it to oscillate.
It is a simple circuit with three inverters in series and the output of the last inverter connected to the gate of the first.
Make sure you set an initial condition and set an initial voltage with the Nodeset command in Spectre. This is needed to start the oscillations.