I have an old road bike I'm restoring--it has friction shifters, a two chainring crank, and a 5spd freewheel/cassette. I'd like to replace the crank as its pretty rusted, and I'd like to use 3 chainring crank to make the hills here in CO easier while commuting.
My question is--how much do I need to replace for compatibility issues? I'd like to leave as much as possible. I'd like to leave the old 5 spd cassette, the old rear derailleur, the old friction shifters. Go ahead and replace the crank and bottom bracket, but maybe even leave the old front derailleur. I've been told that nearly every front derailleur made for a 2 chain ring will work on a 3 chain ring crank, even if a little more effort goes into shifting. Of course, having friction shifters and not indexed shifting makes this MUCH easier.
The oldest crank I can get my hands on are cranks designed for 8 speed rears. or 9 or 10 speed rears. Can I use an three chain ring crank designed for an 8 speed rear? I think I can, but I could be wrong.
In order to go to a triple crank you will probably need to go to a long cage rear derailleur for the extra chain wrap. This can be done with a vintage rear derailleur if you can find one. They can be pretty affordable.
Your 5 speed freewheel will be fine.
Back in the day, I don't know if there was any difference in a double and triple front derailleurs, so you are probably fine there... you may be limited from going super small on the inside ring though.