Hi BB
Quite simple - when intro'ed to service in 95 there was only sufficent Mk3as to equip 2 bases and also provide 1 extra/spare ac when available for training and backup.
Chiv and Watisham were chosen as 2 of the 3 22 Sqn flts to reequip with the SK at the time so Valley took some older Mk3s. The venerable Wessex then left UK SAR service
The differences in the autopilot (which are in many ways considerable), the nav system and at the time the digital radar (now equiping all RAF SKs) meant that aircrew (and groundcrew) training were specific enough to be considered as a seperate evolution with no instant cross over to the Mk3 without relevant cross over training. The handling characteristics in the hover in particular can be very different and the AFCS procedures can vary a great deal too. Clearly the winchmen and some groundcrew trades could swap over and this was done regularly. Otherwise pilots and radops did not swap bases in the early years to keep the shift roster going and needed a short refamil course with the Mk3 before swapping back to the Mk3 and/or going on detachment to the FI. Other than short term detachments the differences in the Mk3a were considered big enough to mean that a permanent move to another base would not be without expense.
We can discuss endlessly the potetnial safety management challenges of having pilots (and radops) multitype rated - but suffice to say that the duplex Mk3a autopilt/AFCS is very good and a major safety improvement over that fitted to all other UK SKs - good as that was at the time.
Cheers