I have a code reader and it told me cylinders 1, 2, and 3 are misfiring. I also have and older battery and have been told it needs replaced because its low voltage. I have also been told I need a timing belt considering my car is at 96K miles. What could be the problem? And is this a problem I could reasonably fix on my own? (I understand I cannot fix a timing belt myself)
If you want to change your battery, that would be easy enough to do. But take note that there may be a radio code you need to know after replacement. Your best bet is to have the timing belt done by your repair shop and have them diagnose the misfires. But before they do the timing belt have them check the integrity of the engine via a compression test so they aren't replacing a timing belt on a bad engine.
Bad ignition coils or spark plugs would be my first guess. Low voltage to the coils or if timing is off could also cause misfires. I just looked up 2012 corolla and you have a timing chain, not a belt. I doubt you would need to replace it. Get your battery tested (many stores do it for free).
If you're mildly savvy with a screw driver, you can swap an ignition coil between one of the cylinders that's misfiring and one that's not. Clear the codes (free at auto stores) and run the car a bit and wait for codes to show back up. If the misfiring code follows to the new cylinder, there's your culprit. If not, could be spark plug. At 100k miles, if you haven't had them replaced that you know of, might be a good idea to change them all.