whats wrong with my betta fish?

Bettas are tropical fish you need to put it in a tank with a filter and a heater. maybe even a bubbler. I would suggest getting a 5-10 gal tank asap. and the heater is the most important for him at this point. but also get the bubbler if you can for more oxygen. The tank will most likely come with a filter.

Just because your last fish survived for that long doesn't mean it's OK to neglect the needs of the fish. 3 years is actually not that long for a betta, mine have lived more than 5 years on average.

You need to place him in a filtered tank. A small 5 gallon tank is compact, and cheap, less than $10. A small filter for the tank will cost you less than $15, and will do a world of good for your betta. They need a stable temperature around 78 degrees Fahrenheit.

An orange coating is often a sign of fungus.

The orange coating is from a sickness. Go to the pet store tell them about your fish they should be able to give u some drops to put in the water. That happens from minerals in the water you get from your sink, maybe get a britta filter, filter the water then put it in. Or use bottled water. DO NOT PUT IT IN A FILTERED TANK! The filter can scare it causeing it large ammounts of stress. To make him warmer Put him near a window, if you live in a colder place like san francisco then rap a small towl around the bottem of the tank. Hope that helps :)