I have to second that cory deal.. I'm sorry, but they may be alive.. but they aren't in the proper tank with the proper environment.
Corys are best in groups of 4/5+, as well as larger tanks (10+ gallons)... the problem with the 2.5 is the lack of actual space (that is a tall tank, the footprint is very tiny for them), and the bio load is too much for a tank that size, especially without a proper filter- the bubble filter, to me, is not a true filter.. it just blows up bubbles. But good at hiding the waste.. sort of.
Again, fish can live in unfavorable conditions, but normally it catches up to them in either illnesses, or shorter life spans, etc.
They sell the tanks with pictures of all sorts of fish on the packaging, but in reality they aren't good for the pictured fish.
Bettas do better in shorter tanks that are longer.. cory need larger areas to swim around on the bottom, with a school of them, and plenty of plants and such on the floor to hide in and dig within.
You are doing a good job, since the cory have been alive for over 2 years.. but it's still not ideal in any way to have all 3 in such a tiny tank.
Nothing we say will have you get rid of them- but it's to educate you that it's not ideal to keep multiple fish in anything under 5 gallons.
Keep doing what you're doing as it seems to be working for the cory- just keep in mind next time you get a tank, or if something was to happen to the cory you have (hope not!) and you considered replacing them. :)