BE36 pilot reported the cockpit door opened inflight resulting in a return to the departure airport.

Narrative

At approximately XA:15am of the morning of DATE; as I rotated Aircraft X on XXR at ZZZ Airport; on my attempted positioning flight to ZZZ1 to pick up an Angel Flight patient; the cockpit door popped open. I informed ZZZ tower that I needed to come around to land to close the door and to try a new take off. With a 500 ft. ceiling; they asked if I had the Runway in sight; I told them yes; that I could stay in the pattern with the Runway in sight to come around and land...which I did with no issues. When I stopped Aircraft X to hold short before the taxi; I reached over; went thru the door closing sequence once again. Ground control directed me to taxi to XXR and contact Tower. I did. While holding short at XXR; I pounded on the door; top and bottom. It was secure. I was cleared to take off. Upon rotating again; the door popped open a second time. When I informed Tower that it had happened again; this time Tower directed me to climb to 3000 for an IFR approach. I asked to remain in the pattern again to land as I had done before. I certainly didn't want to be in the clouds with the door open to attempt an approach to 500 ft. It took 4 calls on 120.9 for them to answer me. Finally; another Operator answered to ask if I had the Runway in sight. I did. After I landed; Ground Control directed me to taxi to my parking area; which was the farthest point on the airport from where I had stopped. I suggested to the run up area to physically shut down and shut the door properly. She acquiesced and I proceeded. Slammed the door this time and pounded on the top and bottom. All seemed fine. Crossed my fingers hoping that 3rd time was the charm. It was. No further problem with the door the rest of the day.