Just as my late addition to the thread. As per Hemo, the mites had likely been made resistant to apistan by the vendor - who is clearly an ***** IMO, to say the least. Needs naming!
On top of that, Apistan is renowned for inefficiency if not applied in optimum conditions (do they still quote efficacies from non-UK tests, like they used to?). Spain and/or Italy IIRC.
That likely accounts for your (expensive) travails, trying to rid them of the pest.
Oxalic acid sublimation would/should have dropped virtually all the mites in the swarmed colony if they were treated before any new brood was capped. Remember that for the future. Likely, too, that the queen was an older one, as well.
You have done well not to lose your colony/colonies since starting. Please go into winter with the minimum of space beside and above your colonies, no holes left open in the crownboards, 50mm (ideally) of effective insulation above the crownboard, mousegards in place and (if well stocked with stores at present) check for adequate stores when brooding starts in the spring.
Not a lot more you can do for them (yes, the extra solstice vape may be good, but should not change their survival chances unless heavily infested at present). Winter bees will likely (unfortunately) already be infected by these residual mites while they were still pupating, so nothing can be done about that.
Good luck with over-wintering all three successfully. There may well be a carpet of dead bees on the hive floors, in the spring, and the colonies weak, but hopefully still strong enough to build up in the spring.
Name that vendor! Or other newbies may well suffer a worse fate than you have done!
RAB