I believe the upgrade path would be this. Upgrade from CCM 3.3 to CCM 4.1 to CCM 4.2 to UCM 6.X. There is no direct upgrade from CCM 3.3 to UCM 6.X. Another path would be CCM 3.3 to CCM 4.1 to UCM 5.1 to UCM 6.1.
However, if you have had a CCM 3.3 for a while, I doubt your hardware will support UCM 6.X. So you would have to buy new hardware, then install CCM 3.3, backup your existing system, restore it to the new hardware and etc.etc.
Unless you have a HUGE system, I would build the new hardware as UCM 6.X and build it up from scratch using the BAT tool to bring in your phones/users. Run it on a test LAN until you are ready to slide it in place. A quick unplug and plug of the ethernet cable and let the phones upgrade their software.
You could do the same with the upgrade path but it's probably almost as labor intensive building a new system as to do all the upgrades and then test everything. Depends on how good you are with BAT.