There's just barely a jump in performance or features over SE 2nd Gen. It's only 1.3x better single core score, 1.4x better multi-core, and 1.2x better graphics over SE 2020.
There's no new hardware besides 5G modem and A15 chip.
So if you can get a new SE 2nd at 30% or less than a new SE 3rd Gen, then I'd get the SE 2nd Gen.
Or even better, just get an 11 at $499 or less -iPhone 11 has 2x optical zoom on cameras, FaceID, ultra wide chip, much larger battery, Night mode for Photos(and also Deep Fusion), U1 chip.
SE 3rd Gen is a huge step back on so many specs. Even iPhone X is a step up on most components (besides SoC and RAM).
When chips are as fast as they have been for the last 5-7 years, upgrades from year-to-year get negligible, even going 2-3 years back won't even be noticeable for most users.
Apple is making everyone overestimate the jumps in performance of its latest chip over past ones and getting so many to put SoC upgrades over substantial hardware upgrades.
A15 won't turn the iPhone 8 into an iPhone 13. It's still an iPhone "8S" no matter what Apple calls it or how high A15 benchmarks go.