Cotyledons are the first 2 leaves you see, basically the 2 halves of the seed. They will die as the plant puts on "true" leaves. Don't waste seed by putting 3 in there, the germ rate on tomatoes is generally high. But if you insist, put 2-3 seeds in the pot and clip off the 2 worst looking ones once they put on 2-3 true leaves. Singing won't help, just like most of the home remedies and "pawpaw did it every year" stuff. If you want more tomatoes, plant more or get better at growing them.