The G26 and G27 were the first Glocks to have the finger grooves on the grip, and as such, would be Gen3. But, since they don't have the mounting rails on the dust cover, a lot of folks want to call them Gen 2.5. However, they simply have no room for the rails, if they did, they'd have them....and would still be Gen3.
Early G26/27's didn't have checkering on the front strap between the finger grooves, but the checkering was added some years into production. Some folks want to call the early one's Gen 2.5 and the checkered one's Gen3, but for my money, a little checkering doesn't make a generational difference.....still Gen3.
For my purposes the G26/27/33's have always just been 'subcompacts', it's easier that way. Now though, with the coming of the Gen4's, I guess the old one's are Gen3's.
Clear as mud, right? And I'm sure someone will chime in to disagree.....