I bought 25 cherry shrimp for my 10G planted tank and since then, a couple of batches of fry (sp?) have hatched so I must have well over 50 in there now. Very low bioload and 0.0 ppm nitrates since the plants are sucking them up so well. I still do 25% PWC's weekly to replace the trace elements and moderately hard water that I have so the shrimp have plenty of calcium to grow healthy shells. As far as I can tell, I haven't had a single shrimp die but if one die, I do have a few MTS (snails) that probably gobble up any carcasses.
Anyhow, my point is, you can get more than 3 if you want. They have a very, very low bioload compared to fish.