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Unless there's something very weird going on, all three flats will be the same leasehold tenure. The two people who own two of the flats also coincidentally own half of the freehold each. Presumably they'd sell you that share at the same time they sell you their flat, but they don't have to.
You'd agree what maintenance needs doing with the other freeholder and then send a third of the bill to the other leaseholder. This isn't terribly different to if all three of you were freeholders.
grams-
Yes, but that's not the concern just the context. The real issue is that there's not been a maintenance agreement in place, and no maintenance has been done for ~20 years.
If there's no maintenance agreement in place how do I and the lender have confidence that necessary maintenance will be organised?
neu
3 flats in the building, 2 of which are leasehold with a share of the freehold to the building. The other flat is leasehold without a share of the freehold.
So 2 different maintenance relationships exist. One with the other freeholder, and one between the 2 freeholders and the leaseholder.