Can I include my point of departure in the three landings for my commercial solo cross-country?
It depends on how you classify the flight. More importantly, it depends on how the DPE classifies the flight. Most people consider any flight with a stop longer than a fuel break as the end of the flight at the fuel stop, and the beginning of a new flight at the subsequent start up. Some sticklers would consider anytime you shut down the engine on the ground, the end of that flight.
You can log it any way you want. I doubt any DPE will count a flight with an overnight stopover between legs as one flight.
Here is an alternative. Takeoff at Airport A. Fly a closed traffic pattern to a full-stop, taxi-back landing. Fly to Airport B where you perform a full-stop, taxi-back landing. Fly to Airport C where you perform a full-stop landing. As long as the straight line distance between A and C is more than 250 Nautical Miles, your total distance is over 300 Nautical Miles, and you are either solo or performing the duties of PIC with an instructor, you should be good for the FAA Title 14 CFR Part 61.129(a)(4)(i) requirements for a commercial pilot.
Note that the regs don’t stipulate a full stop, taxi-backs. If you are flying solo, it gives you time to set up for the next leg. Although it is hard to believe that there is not another suitable airport in 300 Nautical Miles. In what area are you flying?