Glad to hear that you found my article helpful. I would suggest that you write to the course director and explain your circumstances, i.e., that you would like to start Part 3 now and meanwhile prepare for your Part 1 & 2 exam. The intention of attending Part 3 before Part 1 & 2 is so that you can use all the content that you would learn in your Part 3 as evidence for your Part 1 & 2 Prescribed Exam. Accepting such applicants is at their discretion. If you do not get a positive (which I doubt would be the case) you can reach out to RIBA or Chester for their Part 3 course. I did my Part 3 before my Part 1 and 2 Prescribed Exam. So I am not sure why this should be an issue for other applicants.
All the best,
I am sure you will find a suitable resolution.