Hypothetical WAG: if your chainrings are on 5 mm centers, and the chainline for a double is between the two, then when you go triple, you want chainline to be on the center ring. That means moving the set over 2.5 mm. If the spindle is symmetrical, then that means you want a spindle 2.5 mm wider on each side,or 5 mm total. 116 + 5 = 121 mm.

Disclaimer: maybe it's not that simple.