Each holster is molded specifically to that model. A 26 holster is shorter than a 19 than a 17.
They grip on the front of the trigger guard, so a 17 may work for all three, but not a 100% as Glock is known for variations in the molds even when all are the same generation.
The benefits of the subcompact holsters compliment the benefits of a sub compact gun. Less gun and less holster to deal with. One holster for a mid and full size would be OK, but one holster for all three I think you would be selling yourself short.
So, a holster that's made for a Glock 26 will also allow a Glock 19/17 fit into it as well?
From our FAQ:
11) Will my firearm fit a holster designed for a different firearm?
Our holsters are built to spec for each item they are designed to hold. That said, there is some crossover that will allow other models to fit. Typically, a smaller gun of the same make will fit in the holster for the full size version.
Example- A Glock 34 holster will carry a 17, 19, or 26 without issue (as well as the 40SW and 357Sig versions of those guns). Same goes for light compatible holsters, as long as the light is mounted on the smaller gun.
Firearms with a thumb safety can create an issue. A holster built for an M&P45 without thumb safety will NOT fit an M&P45 with thumb safety. The sweat shield of the holster will impede the thumb safety model from being inserted.