The iPhone SE Plus may very well use iPhone 11 (or XR) form factor, 6.1 inch LCD, 4G (LTE), power button TouchID, A13, smaller notch, iPhone 11 cameras, and replace the XR at $499.
Original SE: released March 2016, 2.5 years after the 5s, jumping from A7 to A9
SE 2: April 2020, 2.5 years after the 8, jumping from A11 to A13
The formula is using a hardware design that is no longer current. In fact, a design whose last new incarnation was released 2.5 years ago. Add the current SoC, which at this point because we are mid-cycle will be six months old and thus also two generations newer than the last release in this hardware design. After the 5/5c/5s design used for the SE, there was the 6/6s/7/8 design used for the SE 2, there comes the (X/Xs/)Xr/11 design. Last release with this design was the iPhone 11, released in fall 2019 with the A13. Therefore, the SE 3 should come out in spring 2022, using the iPhone 11 case and jumping from A13 to A15.
But of course, Apple reserves the right to break pattern (it broke pattern with Xr, also when it added the 12 mini). So, it could come out a year early. But that would complicate the lineup. It would need to fit in between the $399 SE 2, the $499 Xr, the $599 11 and the $729 iPhone mini price-wise, while sharing the A14 of the 12 mini. This doesn't make sense to me. Putting the A14 into the iPhone 11 would be weird, that's too close to the iPhone 12 (the latter just having OLED instead of LCD and one-year newer camera system). Putting the A14 into the Xr would create something that both above and below the 11 (CPU above, camera below). I'd say the Xr and 11 already fill the space below the set of iPhone 12 series phones pretty well.
If it follows the original pattern, the fall 2021 lineup would be:
- iPhone 13 Pro (Max)
- iPhone 13 (Mini)
- iPhone 12
- iPhone 11
- iPhone SE 2
In spring 2022, this would change into:
- iPhone 13 Pro (Max)
- iPhone 13 (Mini)
- iPhone 12
- iPhone SE 3 (iPhone 11 design)
(maybe still keep the SE 2)