Hi SeeBee, and welcome to Tropical Fish Keeping forum.
Sad to say, some so-called pet stores have as their first goal selling fish and supplies. The best advice I can give is for you to thoroughly research before entering the store. The members here will not mislead you.
I completely agree that a 3g is not sufficient space for most of the "common" fish. And the cycling, which takes 2-8 weeks, will certainly negatively affect if not outright kill any fish.
My first suggestion is to return the Danio, they are not suitable for that small a tank and they are going to die, i can all but guarantee it.
Here's an article on cycling, it is one of the "stickies" as we call them at the head of each section of the forum: http://www.tropicalfishkeeping.com/f...m-cycle-38617/
This sticky on fish stocking might also be helpful: http://www.tropicalfishkeeping.com/f...tocking-38626/
For a 3 gallon tank, you have some options. First, a single fish like a male Betta. Live plants will greatly help. The water movement issue mentioned is important however, we could discuss this later. Second option, a group of very small "dwarf" fish, such as Ember Tetra, Dario dario (Scarlet Badis), Mosquito Rasbora and similar species. A group of 6-7 would be OK with some plants. You will note these names are shaded; that means we have a "Profile" on the species, and clicking on the shaded name will take you there.
Profiles are under the second tab from the left in the blue bar across the top of the page. You can browse these to get more ideas about fish. Minimum numbers for each species (shoaling fish need a group), water parameters (some fish adapt somewhat, others don't), and minimum tank size for the group are included in the profile.