If you are enumerating subject-action sentences your last delimeter is 'a' in most cases: 'Паша пишет, Саша читает, а Коля отдыхает'. It is just enumeration. 'a' can be omitted, but sounds good, natural. I think it can be translated as 'and' or 'while'.
If you are using 'но' as the last delimeter you oppose last sentence to all others: 'Паша пишет, Саша читает, но Коля отдыхает'. Something like 'Look at Kolya! What is wrong with this guy?'. I think it can be translated as 'but'.
These rules apply to 'subect-action' sentences only.