every 20mm strap is compatible with Watch 3 41mm
every 22mm strap is compatible with Watch 3 45mm.
so basically it depends on the model, and from my experience, I advise buying straps with quick release watch spring bars. (ps. you have many many options on Aliexpress if you do not mind whether it is original Samsung or not, but the quality is great, I have many straps from Aliexpress and they are good quality especially for that price)