This definitely can work (I did it with a Superhub 1 as a rainy day project), but set your expectations to low. The wifi quality of most ISP hubs is low, and the Hub 3 and SH2 aren't designed to work together as a mesh system, so you need to manually manage network names and channels.
Personally I'd always recommend biting the bullet and buying either a cheap wireless access point if you don't need smooth and continuous wifi coverage, or a mesh system (with Hub 3 in modem mode) if you do want excellent smooth coverage. I plumped for a TP-LInk Deco M4 mesh and it's been superb. And in most situations you won't need to link secondary mesh units up by ethernet whereas you will have to with a recycled SH2 or a wireless access point. The practicality of long ethernet runs depends on your circumstances, but they can be a right pain to do neatly, safely (mainly trip hazards) and securely.
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