It occurred to me that the unused gliding sprites may have been intended for Sonic 3 rather than Triple Trouble as well. Specifically the two closest to the left of the final(?). I can't think of where they would have fit in TT, but in Sonic 3 they'd make perfect sense as part of the begging of Sky Sanctuary Zone where Knuckles glides to the switch after he's worn out from fighting and electrocution. The second to the right--also unused--eludes me. Maybe it's for when he's caught by Eggrobo before the Mecha Sonic fight. Considering TT came out between the two halves of Sonic 3, makes me think SSZ was planned pretty early on?