my tank is 3ft long and 1/2 foot high and has a width of 1/2 foot how many three inch fish could i get in it

You could two get goldfish in a tank that size easily although you aren't recommended to do so. Just make sure you have a good filter and you keep it cleaned and your fish should live happily.

Are you sure about the dimensions here. Thats a strange tank. Most tanks the side glass is 12" wide.

If the dimensions are right you have a - 5.6 US gallons tank this isn't big enough for one goldfish. This would be better sooted to be a Betta tank.

Are you using a rubbermaid tub as your tank? If so that is perfectly ok but you might want to spend about ten bucks and get a bigger tub that you can actually see your fish in. Good luck.