I have a 3 gallon tank and i just added the following fish: 3 neon tetras, 2 male guppies and 1 black fin sharks. what are your guys' thoughts? The tank has a heater and filter and there are some decorations such as a bridge and some plants.

Way overstocked. Black fined sharks get 12-18+ inches long and need 75+ gallons when full grown. Neon tetras need groups of 6 or more. Neon tetras and guppies need at least a 10-20 gallon tank. You need to get a larger tank asap before you stunt your fishes growth and or kill them.

Edit:
Yes, they would die if you left them in the current tank. Black fin sharks are also brackish fish. Guppies and neon tetras are fine together but the shark as it gets bigger could eat them.

I highly suggest you return the shark and then get a larger tank for the guppies and tetras.

I have a 2000 gallon pond with 16 goldfish pets question and answers,www.5d2d.com
Research

way too many fish, i have 3 oscars, 1 convict and 1 pleco in a 55 gallon tank, thus there ok now since there all under 5 inches, but even i will have to purchase 150 gallon tank in the next few months. should have done some research before you went out and bought it all.

my thoughts are they will all die without a larger tank. neons like to live in schools of 5 or more. so with three they are stressing a bit. black fins need a much larger tank. 3 gallons is too small for a betta to live happily. you need to look at snails, shrimp or african dwarf frogs for things tiny enough to live in a small tank. if you want to keep the fish you have get them a bigger tank asap. and some bio-spira to cycle the tank a little faster.

*cringes* That is really a terrible tank setup, no offense. It's not your fault. You just really can't trust those people in the pet shop as they know nothing and are just out to sell things. On the other hand, some are really knowledgeable. I go to Big Als and the people there know a lot about fish and tell you what's really best. Best thing is to do your research before buying anything. The person who replied earlier was absolutely right. That tank is waaaay too small. In fact, a 3 gallon is too small to carry any type of fish in this world! Maybe some snails or shrimp, but even a betta would be cramped in there.

pets question and answers,www.5d2d.com

Anyways, the only thing to do is UPGRADE. Get at least a 10 gallon and bring the shark back to the pet shop or get rid of it somehow (without killing the poor thing). If you want to keep the shark you need a much larger tank. If you leave them in there, they will probably die after only a little while. They might survive, but wouldn't you rather they thrive?The shark will become stunted, which means there isn't enough space to fully grow! Imagine a human baby having to live in a tiiiiny closet! At first, yes they may fit... but you get the point right? Plus that tank cannot handle that much waste. You'd have to be doing water changes each day, if not more. Please don't make that mistake!
Goooood luck

Okay, if you didn't have the black fin shark. Are you crazy? Give it back to wherever you got it. These things grow up to 18" long. The size of the fish which you eat. I mean it. Look it up on google. You'd need an aquarium the size you'd find in zoos and underwater worlds to keep these. If you keep it, it'll just suffer from stunted growth. Some fish are like that. If the tank isn't big enough, they don't grow properly, but some parts keep growing and the whole fish becomes like mutant and dies a slow, horrible death. Of course, some just keep growing until you can't stand it not being able to turn around itself and take it out to either give it away (though who has a tank so big?) or to kill by freezing. Better to give it back to the petshop they'll know what to do with it or maybe some rich guy with a huge tank will come along and buy it. pets question and answers,www.5d2d.com

Other issues: It's better to have a species only tank. Either all-guppy or all neon tetra. Haven't you heard of compatibility? Neon tetras tend to be nippy fish, so they'll bully your guppies by biting them. Bad. Your guppies will die. So when you go back to the pet store to return the shark, return the guppies/tetras as well and exchange them for more neon tetras and a up to four cories or loaches (tell them to get the smallest kinds and tell them how big is your tank).

So new list:
-up to 5 neon tetras/guppies (pick one species only!!!)/or 1 male betta fish (bigger and more beautiful but will fight with others in such a small space)
-up to 3 loaches/cories (bettas leave these alone, but if you want these, it's better if you have a bigger tank).