Welcome to the forum. I'm relatively new here as well, but I am experienced enough to be able to offer the following advice: Listen to Byron!
I can't stress that enough.
To that end, I would like to suggest a course of action. I'm sure it's not the one you were hoping for, but I do think it will pay off for you in the long run.
1) Return some fish to the store where you got them. As Byron said, the Danio will outgrow a 15g very quickly and will not be happy in such small numbers. Same for the tetras. 3 individuals from different species will probably be very stressed and become susceptible to disease. Maybe choose whichever of the 3 tetra species is your favorite and pick up another 5 or so of that kind? If a clerk there advised you to stock these fish in your tank in these numbers, you are within your right to expect a refund.
2) Buy a floating plant! They really help to stabilize water conditions and the fish really love it! I wish I'd added floating plants even sooner than I did. Water Sprite
or Brazilian Pennywort
are the two I see recommended most frequently, though many others will do a great job if you can't find those.
3) Get a timer for the light. You can find cheap analog ones for a few dollars that will do the trick. The eyes of fish have evolved to adjust to slow, gradual changes in light level (i.e., dawn and dusk). They can become very stressed when thrust from bright light to complete darkness and vice versa, often swimming frantically into glass and tank decor. Set the timer to turn the light on at least 1/2 hour after some ambient light enters the room (indirect sunlight, room light, etc.) and at least 1/2 hour before the room becomes dark.
Good luck and keep us posted!