All that's available now is 4/5dr sills, that are outer skins, the rear has no reinforcement like the 3dr ones so that will almost certainly need made up to keep the strength under the rear quarter.
From the b pillar forward the sills are identical, I'm just using the last set of genuine 3dr sills that I know of on a restoration project and they were 300gbp for the pair...
Also it seems to be a struggle to get the 4/5dr sill now, the set I used earlier this year took 5 weeks to arrive, the last set that was ordered around 3 months ago still haven't arrived....