The granny is just about always a different bolt circle than the other two. 110/74 is a typical example.

So if you want to make a triple to a double using the existing outer rings, you won't be able to move them inward. You can simply drop the granny, and but a bash guard made to go outside of the outer chainring. Otherwise if you want to use the three existing positions, with the bashguard where the outer now is, you'll need to replace the (now) middle with one the same size but with the graiiy's Bolt diameter.

Just so you know, many bikes have the chainstay angled as it passes by the crank, so while a 26 or 26 will clear as a granny, often a 34 or 36 won't. (my commuter is that way and the 26t and 36t both just clear the stay with 1-2mm to spare).