Big Sainsbury's in Triangles? Since when there's a big sains there? The big Sainsbury's is on Whiteladies Road, aka Clifton Down. The one on Queens Road (ie near Triangles) is not that big, but big enough to get most of your groceries.
The massive 24h Tesco too far from the city centre (unless you take a bus there). The nearest Tesco (Tesco Metro) big enough for your groceries shopping is in Broadmead, not far away from the coach station. Tesco Express is crap because everything is more expensive there.
There's one Somerfield (quite small) near Tyndalls Avenue (the main university road). I don't buy my groceries there; I would rather walk an extra 5-10 mins to the big Sainsbury's in Clifton Down.
There's an M&S in Broadmead as well, if you're an M&S kind of person.
There's one Asda about 20-30 mins from city centre (near the train station) but the big one is in Cribbs (you need to get a bus there).
Hope that helps. =) Otherwise, use the store locator on the supermarket websites.