That's a standard size 120mm pc fan, they aren't 'so small', small would be 40mm or even upto 80mm lol.
The reason you can't find them single is that even TT doesn't sell them single, they only come as optioned as a 3pack. TT direct is actually the cheapest source by $4. You'd think that being designed specifically for radiators they'd come in 1 or 2 or 3packs for the 120mm, 240mm or 360mm AIO's but oh well, TT's decision there.
You can do as you please with the other 2, many will use them on the cpu cooler, on the top, even as additional bottom front intakes or do nothing but stick them on a shelf, just incase one fails down the road. Just try not to stick fans where they'll actually disrupt your air flow through the case...