I have converted bikes with 2x5 and 2x6 gearing to triples, with good success. You are most likely to have problems with large-large crosschaining, which is why you want the small chainring as close as possible to the chainstay and the small freewheel cog as close as possible to the dropout (allowing for chain rivet clearance, of course).
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"Far and away the best prize that life offers is the chance to work hard at work worth doing." --Theodore Roosevelt
Capo: 1959 Modell Campagnolo, S/N 40324; 1960 Sieger (2), S/N 42624, 42597
Carlton: 1962 Franco Suisse, S/N K7911
Peugeot: 1970 UO-8, S/N 0010468
Bianchi: 1982 Campione d'Italia, S/N 1.M9914
Schwinn: 1988 Project KOM-10, S/N F804069