Welcome to TPT.
Here is a good place to start: https://www.plantedtank.net/forums/8-...ml#post1960797
Unfortunately, small tanks can be hard to keep because small chages get amplified in small volume of water.
I would start with:
- sand substrate: pool filter sand, Black Diamond from Tractor Supply are cheap and don't affect your water
- a small HOB filter like https://www.amazon.com/AZOO-AZ13097-...ds=Azoo+filter
- cheap and simple
- plants need light to grow, any simple desk lamp with CFL bulb like https://www.homedepot.com/p/EcoSmart...450K/205487810
work. Table lamps with adjustable height are good as they will allow to control the amount of light
- most tropical fish need water temp in the 74 -84°F range. Finding a heater to fit a 3g tank is almost impossible. Go without heater
- fish: check out Endlers, widely available, small, do fine without heater, and are fun. I would start with 2 (after the tank is cycled)
- plants: elodea, hornworth, Java Fern, a gazillion types of moss would be a start. Check out TPT's subforum https://www.plantedtank.net/forums/156-wtb-raok/
and you might get a bunch of plants for $10.
Learn as you go, ask questions, and just enjoy.