Colonoscopy prep, vomited, fainted, fell, head injury, ER & for 2nd colonoscopy same + knee surgery from fall. Time for 3rd, please advise.

Hydrate well. Given your previous issues make sure the indications for your colonoscopy are sound. Hydrate well pre and during prep, use non osmotic prep eg not fleets phostasoda or visicol but go lytely or MiraLAX (polyethylene glycol) type prep, do prep over slightly extended time period and have someone with you...Don't stand suddenly...