First off, stocking issues: neon tetras cannot live in a 3 gallon, and need at least 8 to thrive. Same with tiger barbs. Danios also cannot live in a 3 gallon, they need lots and lots of room to swim. Get rid of the neons, barbs, and danios.
Are the guppies both male? If it is male and female they will breed like crazy and overstock the tank. Also guppies need hard water, not soft water. It would probably be best to give them somewhere that has hard water.
Did you cycle your tank before adding fish?
Do you have live plants?
Do you have a filter and heater?
What do you mean when you say "I use drop to remover chlorine"?
The tank is way overstocked, causing the fish to die. Any amount of nitrite is harmful to fish, how much nitrite was there when you said "barely any"?