I assume that as the System 3 was a similar size (3U high) this used the same Card Frame as the System 2.
The Card Frame was 3U high, 84HP wide and 180mm deep (1U = 1¾" or 44.45mm, 1HP = 5.08mm).
The left hand position is to take the PSU module (14HP wide), and an 8-slot Backplane provides for 7 5HP wide Front Panels. On the right hand side is a single 35HP wide full size 5¼" Floppy Disk Module
Chris's Acorns has a photo of a 6809 variant System 3, I only assume the standard 6502 based System 3 was similar although this example does not have a PSU module fitted on the left hand side, its probably mounted to the rear behind the Card Frame.