Converting duble to triple crank
 

I know how to install the new crank, but my question is-how do I know where to set the derailleur cable and the height of the new triple derailleur? I have friction shifters presently on the downtube. Currently has 2 rings in front and 8 cogs rear.

the height of your dérailleur should be 2 mm above your biggest chainring. have your shifter in the lowest gear, pull the cable taught,and tighten the bolt.

Cable doesn't have to be really tight, just taut. No slack.

If the triple crank has the same big chainring as your former double (say 52T) then the fd height won't have to change at all. You will probably have to adjust the limit screws to allow for the longer bb spindle and the wider throw to let it reach the granny ring.

If you are buying new parts there are directions included. Park tool website is good for knowledge too.