Because of the convention for lower-case tags, i or ii or iigs doesn't look great and isn't intuitive.
The most concise and readable tags are: apple1, apple2, apple3 and apple2gs.
Besides, there is a precedent for apple2gsapple2gs.
UPDATE: Here are sample tags.
commodore-64 amiga-500 zx-spectrum pdp-6 (used now)
apple1 apple2 apple3 apple2gs (my original preference)
apple-1 apple-2 apple-3 apple-2gs (with hyphens)
apple-i apple-ii apple-iii apple-iigs (these are more consistent - vote here)