i messed up a lot will my fish be ok?

[Noah]

ok a few days ago i bought a fish tank 3 gallon with a bubler,fake plant, some rosks,a cave thing,and 3 fish all this from walmart (shut up) when i got them all home set up the tank, and put the fish in imidatly for fear that they would sufocate somehow from reused water. anyway the fish i got were a green spotted puffer, fruit tetra and a mollie and when i bought them i thought that only betta fish were mean to other efish. since i knew that puffers were sorta special i figuerd i should research it and when i did ho-ly crap i found out i did tons of things wrong first i did not know about the nitrogen cycle thn i found out that puffers are mor suceptale to disease cus they have no scales. that night my fruit tetra died. probly because the puffer will only accept certain fish. then 2 days later i found out it is necicary to et a filter so i got that,while i was there i found some test strips for things like amonia. and some plant seeds. so i used tat stuff when i tested the water everything seemed fine so i thot the cycle had not begun yet but the water was murky. then i read that puffers and mollies were brackish/saltwater fish and tat it was ok to use table salt so as a test i tried to put a pinch of salt ino the tank then i read that you should never evereverever do that you need to put it in a buket and distribute it slowly over a few weeks so now i am just feeling horible but everytime i do something wrong it seems like my fish eithere become more active or stay the same please i need to stick with this so returnng them is not an option i just ned help i am getting a heater tomorow please help me