I would def contact Glock for clarification.

The Glock 25 upper, although similar in size to a Glock 19, operates on a straight blowback system, unlike the locked-breech system of a 9mm Glock 19. This design difference often makes the slide and barrel tolerances incompatible with standard 9mm Gen 3 frames.
Why it fits the Gen 4 but not Gen 3:
Recoil Spring & Dust Cover: Gen 4 frames have a wider dust cover and a larger hole at the front of the slide to accommodate a dual recoil spring assembly, which the Glock 25 often utilizes, whereas Gen 3 frames have a narrower area.
Slide Geometry/Barrel Lugs: The locking block cuts on a .380 blowback (G25) barrel are generally different from a 9mm (G19) locking block, which may lead to it not seating properly on a Gen 3 frame, which has a stricter tolerance for this specific geometry.
Fitment: The Glock 25 slide is designed for the blowback setup; the Gen 4 frame tolerates the slightly different recoil assembly and barrel lug geometry better than the standard Gen 3 frame, which was engineered purely for 9mm locked-breech.

Good evening to you!! The Glock 25 primarily fits Gen 3 Glock 19 frames. Because the G25 is essentially a blowback-operated .380 ACP version of the G19, it shares the same compact frame size, allowing for direct compatibility with G19 Gen 3 frames, as well as many aftermarket, Glock-pattern frames that follow Gen 3 specifications. Hope this helps. Have a good rest of your day.
🇺🇸🇺🇸 'Merica' 🇺🇸🇺🇸

Hi Robert, you can use this upper and magazines on Gen 3, 4 & 5 Glock 19 compatible frames to shoot .380 caliber instead of 9mm without having to swap out any internal components.
Compatible frames include:
Glock 19 Gen 3, 4 & 5
Glock 19X
Glock 45
Glock 47
Glock 49
Stay safe & have a great day!

Hi Robert and welcome. The G25 (.380 ACP) uses a direct blowback system, but the external dimensions of the slide are supposedly compatible with Gen 3 G19/G23 frames. Glock will definitely need to confirm. Cheers and stay safe!