Hi Harold,
You can use a gpio in PORTA group as swith1, and use a gpio in PORTC group as swith3. They are different isr source.
OS will not affect your interrupt function. All the unused interrupt address in the vector table is announced as [weak]. Which means if you announced you own interrupt service function, the default interrupt address is ignored. So if you received an error that the PORTC is redefined, I think this may cause by improper code or multi defined code.
Regards
Jing