Triples have been around since the 5spd era. Your triple is probably 8 or 9spd but should work with a 5-6spd chain. Your FD may or may not have the throw to cover a triple, some do, some don't. Down tube friction shifter should cover the range ok. Chain alignment is not likely to be a problem but you may not be able to cover the big-big or little little ends without some chain rub. Depends on where the BB puts the small ring, which should be within 3mm of the outside of the BB shell.
Steve