At KPHX, there's a Runway 25L/R and a runway 26. What would the reasoning be for using this instead of what seems to be the more common L/C/R designation?
There is flexibility on the numbering of parallel runways based on a number of factors as described in FAA advisory_circular/150-5340-1J -page 4 (see excerpt below). In the case of KPHX for example, there is the main terminal between runways 8/26 and 7R/L-25R/L. Also, when approaching from the east the threshold for runway 25L is a significant distance from the threshold for runway 25R (about 2500 feet inset).
I would guess that minimizing possible pilot confusion factored into the decision not to have a "center" runway designation.
While it is going to be very hard to pinpoint the exact reason for KPHX, there are several logical reasoning for why it is done in general (I must disagree with your observation that XL/C/R is more common than XL/R, X+/-1 configuration, especially when there are more than 3 parallel runways).
Avoiding confusion. During rush hour it might be too confusing to have 3 runways with the same number
Separate complex. There is a good 1KM (3,500 ft) between 8-26 and 7L-25R centerlines. Using a different number helps to artificially (or not so much) separate the airport to different areas (some airports literally use N/S or W/E "complex"/"area" on the official charts)
Preparation for the future. If KPHX is ever going to get a fourth parallel runway to the north of 08-26, it will be much easier to add L/R to the existing runway than renaming the supposedly 07C-25C