I'm curious about the Mark3 HD 3-9. Its greatest asset is the weight at 15.6oz. It's SFP, but for this mag range, it works. Elevation and windage are 15 mils, FOV is about 100ft if calculated at 1x. And the Firedog is pretty bright. I'd think for you purpose of shooting to 300 yards, this would work well. IMHO, the 4-12 doesn't translate as well. It's eye relief has almost 1.25" in variation from low to high.
I think the closest option to the Mk3 3-9 is the Trijicon Accupoint 3-9 at 13.4oz
There are ther options that are in this mag range, that are popular, that fit your budget, but are heavier. Also, while many will be illuminated, none will be as bright as the Firedot.
Primary Arm GLX 2.5-10 at 22.5oz FFP
Primary Arms GLX 4-16 at 23.5oz FFP
Trijicon Credo 2-10 at 23oz FFP
Trijicon Credo 3-9 at 17.02oz SFP
Trijicon Ascent 3-12 SFP 17.8oz, non illuminated