Zebra danios are egglayers, not livebearers. While they may spawn (assuming you have at least one male and female in the trio) it is unlikely the eggs would hatch or if they did the fry survive. Most fish will readily eat the eggs and fry unless both are very well protected (thick plants, large tank, etc). Danios are easy spawners but very prone to eating the eggs; using marbles for the substrate has often been suggested to protect the eggs which fall down and can't be reached by the fish.
I have various characins (tetras, pencilfish) spawning in my aquaria regularly, and I do sometimes see fry that survive predation, but I have large aquaria thick with plants and wood that offers good hideouts for some of the fry. It's usually the eggs that are eaten, as all fish will readily devour eggs if they come across them.
To answer your second question: If you were to raise the fry to decent size, many stores will accept them, sometimes in exchange for supplies.
A last thought, read the article on cycling at the head of this section, here's a direct link: http://www.tropicalfishkeeping.com/f...m-cycle-38617/
Edit: I see kymmie responded while I was typing. B.