Like Andrew Stewart said earlier; the IMPORTANT thing is to be sure that the small inner chainring does not hit the chainstay.

I've converted many double chainring road bikes to triple chainrings and i've often run ito the problem of the smalles ring on the triple hitting the chainstsay because the indent on the chainstay (it there was one) was too small ie short forthe smaller inner triple chainring to clear. there were then a few choices.

#1. Sometimes reversing the bottom bracket spindle would give enough clearance.
#2. Sometimes if there were enough threads on the adjustable cup a shim could be placed under the FIXED cup to move it ouward a bit.
#3. buy a new spindle or cartridge BB with the needed spacing for the smalest chainring to clear the chainstay
#4. Very carefully create an indent in the chainstay for the smallest chainring to clear.

Be aware that tightening the crankarm to the final torque can move it inboard a fair bit. Thus slipping the crankarm onto a spindle to see if there's clearance for the chainring many not work when the crankarm fixing bolt is fully tightened.

In severe cases where the person I was fixing a bike for didn't have the money for a part or if the part wasn't available i've had luck getting the needed clearance by cyanoacrylic gluing pop can shims onto the flats of a square taper crank.

Cheers