Yeah, you're right, the square wave LFO is the best clock source in the Pro3. Assign an LFO to a CV Trig Out and turn the amount all the way up, and you're good to go.
Other fun things you can do, related to clock:
* You can assign more than one LFO to the same CV Trig Out to get interesting polyrhythmic effects
* You can assign an oscillator output to a CV Out into a module that can be clocked at audio rates (many sequencers, etc.)
* Don't forget that Osc3 can be an LFO, giving you a million ways to play with clock timing
* Use CV Outs instead of CV Trig Outs with multiple LFOs at lower amounts. For example, you might find that it takes an amount of 20 to trigger your module. So if you have two LFOs at different speeds, each at amount 15, the module won't be triggered unless BOTH LFOs are in the high part of their duty cycles, again producing some interesting rhythms.