I doubt any inside museum type places will open in tier 3 areas:
Regs:
"indoor attractions at mostly outdoor entertainment venues must also close (indoor shops, through-ways and public toilets at such attractions can remain open). This includes indoor attractions within:
zoos, safari parks, and wildlife reserves
aquariums, visitor attractions at farms, and other animal attractions
model villages
museums, galleries and sculpture parks
botanical gardens, biomes or greenhouses
theme parks, circuses, fairgrounds and funfairs
visitor attractions at film studios, heritage sites such as castles and stately homes
landmarks including observation decks and viewing platforms"