"Male scarlet badis was dead on a leaf in the tank 10 min ago
All of my parameters are excellent, it's the fish that are driving eachother nuts."
Clown Killifish male
Panda Guppy Pair
Scarlet Badis female
Sparkling Gourami Male
Fancy Plecostomus baby
Australe Golden Killifish female
Malabar Pea Puffer Pair
That's the right attitude to take. So while this may be harsh, try to take it in the best and most constructive way possible.
I once overstocked a tank too, and I've never felt worse about the Bolivian ram I killed.
First and foremost, I don't know how you cycled, or when you cycled, but you say your parameters are perfect. On other sites more devoted to fish, this wouldn't be acceptable. You should post detailed parameter checks and exactly how you checked them.
Next... On stocking in general. Wow -- you HAVE to read everything you can about species of animals you buy before you buy them. I took four months to prepare myself and a tank in the best way possible for crystal red shrimp. I read literally every web page in the top 50 google pages and almost every thread I could find on them. These are your pets and are a major commitment.
Next -- on deaths. When something dies you HAVE to assess everything in your tank. Was the death natural? What can I fix. None of the deaths in your tank were natural, I assume. They were all stress related. Fish don't jump usually without some reason. Some do, granted, but yours almost assuredly were stressed to death.
Next -- tank size. Three gallons is tiny. When you consider the millions of gallons these fish naturally live in, it should be a scary thought to have so many in such a small containment. Even bettas which live in "tiny rice paddies" (which for some reason merits their captivity in tiny cups in stores) are living in thousand gallon rice paddies.
Next-- compatibility. With research I mentioned earlier, you'll find that certain species are never meant to live together. Bettas rip up things with flowy tails, puffers are territorial, guppies breed out of control and are relatively stupid when it comes to territory, scarlet badis are super sensitive to water parameters and stocking levels, clown killifish need a minimum of 20 gallon tanks, pencil fish need surface area galore to be happy and every fish-type bottom feeder known to man is too big for a 3 gallon, let alone a Pleco, which even if a BN Plec will crazily outgrow a three gallon.
Each tank you do you should have stocking in mind before hand, check it over with people who know about fish, and then proceed to stock based on parameters, space and research.
All my knowledge of fish comes from glancing at threads, liking a fish then googling it.
I advise you to return everything but 3-6 sparkling gourami and some cherry shrimp. That's a perfect and well balanced 3G tank.