The exams may be completed over six sessions, not three. From the U.K. CAA website:

"EASA regulations state that all theoretical knowledge exams for a particular licence must be completed in six exam sessions. In the case of CPL, IR and ATPL exams, this means completing them within six attendances at CAA exam centres."

The maximum of three sittings refers to the number of re-takes you may have with each subject.

Here is the CAA website for your own verification:

https://www.caa.co.uk/General-aviati...-examinations/