Some cranks with third ring provisions might also need the proper bolts and bushings to properly space the thrid ring away from the middle one. Crankset makers usually provided these if it is needed for their cranks, but sometimes the bike manufacturers do not bother to include them when they sell a bike. No problem back when the bike was a current model, but as the years go by, the bushings and bolts might be harder to find. Your crankset might not need any special adaptor bushings and bolts, but it is still something to watch out for when trying to add on a third ring to a crank.
I have a nice NOS Stronglight Delta crank with threaded holes for a third ring at the back of the spider........Haven't been able to add a third ring as the bolts and bushings for the crank seem to be quite unobtanium these days.....