Like Steve, I have a split system with four wheel disc brakes, proportioning valve and no booster on my BN4. My front calipers are Girling 16s so they fit BJ8 pads, but they appear to be from a TR6. The rears are Jaguar. The discs and mounting brackets came from Denis Welch and no modification or replacement of my hubs were required.
The front brakes on 3000s, except BJ8s are Girling 14s. On BJ8s, they are Girling 16s, with a different mounting bolt spacing. TRF1's Dunlop discs on his BN4 were a period, factory special order item, and not used on any other Healeys, except the 100S.
With the three-pot fronts, I doubt you would need a booster, since neither Steve or I use one. IIRC, Steve has three-pot Toyota calipers in front.
With Jaguar three-pot calipers, the first question is the mounting bolt spacing to determine a mounting bracket. If they fit your 3000, great. If not are they the same as a BJ8? If not, then custom brackets would be required, or find different calipers.
IIRC, Steve makes brackets for the rear Jaguar calipers.
Hope that helps.