What a lovely reason to get your flock
I used to keep a small fleece flock of wethers for a number of years, purely for their special fleece. They do get a bit fat, but they don't mind if you don't.
Breedwise, unless all your weaving will be uniform, why not have a few from different breeds? This would widen your choice. I haven't done enough weaving yet with enough different breeds to advise on that, but I'm currently experimenting with blending silk with Hebridean and Shetland fleece, with lovely results. This might add strength to your weaving, softness and drape. Depends on what you are producing of course.
Once others have come up with some suggestions, why not buy fleeces of possibles to try them out, before plumping for actual animals?
My fleece flock was for spinning and knitting, but I'll tell you the breeds. There was a pure Jacob, Fred, chosen for his lovely fleece from among the flock we had back then. There was a white Shetland, (Cuthbert) ditto, but many Shetlands have lovely fleece, whereas it's more difficult to find a suitable Jacob. We had a Polwarth/Dorset/Ryeland ewe (called Spinning Jenny) with the most wonderful fine, crimpy, white fleece, who we crossed with a Shetland and a Jacob. The male lambs were castrated to join the fleece flock - the Jacob X (Muffin) was mostly dark brown but with white splashes, and the Shetland cross (Jumbo) was white with a fleece so huge and fine I sold it in two halves. Their female twins (Maddy and Mumbo) were kept for breeding to produce more interesting crosses. We had a Gotland ewe we put to Shetland tups, one white, one coloured, and her offspring were either coloured or white. We kept Garbo as a wether and Greta for breeding, and a white male whose name I have forgotten
. All that is to suggest that you don't need to keep all one breed of fleece sheep.
The fleece from wethers tends to be a bit better than that of ewes which are lambing each year, but the plus side of keeping interesting ewes is that you can play around with producing brilliant crosses.
Different breeds will do better in different localities. For example Gotha the Gotland ewe, never produced a useable fleece because hers always cotted before we could get her shorn, although her cross lambs had no problems. Gotland fleece though would be wonderful for weaving.
If you know nothing about keeping sheep, then learn that first, with a small flock. I would suggest you learn to hand shear, as relying on a shearer has problems. They tend to machine shear (which is usually to clear the sheep, rather than to preserve the fleece at its best), and they come when they can fit you in, not when the sheep are ready, and for quality fleece you need to harvest it at just the right time.
Of the three breeds you mention, Bowmont is the finest, but perhaps not ideal for warp, and most difficult to source, BFL is variable but soft and a bit higher micron count than Bowmont, and Shetland is extremely variable - best to choose stock from a breeder who breeds for quality fleece. First shear fleece tends to be finer and 'nicer' than older sheep, but with very old sheep the fineness and softness can improve.
Just some thoughts.