It's a great idea to work out a health plan for your flock with your vet
It sounds as if there must be a lot of wormer resistant worms in your area for her to be so hot on wormer rotation. It is certainly good practice/vital in a large flock, but while you have only three sheep, on clean ground, and presumably with no evidence of wormer resistance, it seems a bit OTT.
I agree that ALL sheep brought onto your land should be wormed with the new class of wormer (Zolvix) to blitz anything there, and kept in isolation. I would say keep them on hard standing for 48 hours til all worms and eggs passed (burn straw) but also to isolate them from the rest of the flock for 2 to 3 weeks, to check they are not carrying any other illness. That way you have some chance of keeping your flock fairly worm and disease free.
The danger is always bringing new animals onto your land. We have got to the point where we run a closed flock, so our worming can be kept to a minimum. Older ewes become tolerant of a small worm burden, but obviously we keep a close eye on our lambs. We worm the lambs once at about 2-3 months, depending on need, then just pick out any poopy bums and worm them as needed. The meat boys are then done again in the autumn and if necessary about a month before slaughter, so in July.
Our vets are always happy to open bottles and split, to make sure that all small breeders in the area are keeping healthy flocks. If your vet won't do that, there might be another in the area who will. Using very out of date meds means that they might no longer be effective, plus when you get a visit from Trading Standards they will pick up on that.