I built my own trailer specifically for the Mk3, so I'm not sure how helpful my info will be.
The tubes are directly supported by bunks that tilt slightly inboard. The keel is supported by two additional bunks in a fairly tight v configuration. The keel bunks slot between the speed tubes so that it automatically centers the boat up even on a choppy beach retrieve.
It took me quite a lot of trial and error to get the bunk heights and angles right. The boat was probably on and off that trailer a dozen times before finally welding the mounts on.....and the only time I have used the transom wheels lol. If you would like any angles / measurements, just give me a shout.
It's well worth the effort in getting it spot on. You tend to see quite a few trailered SIBs with kinks and bows in them after they have lived on a badly fitting trailer for a while.
I knew I had a photo somewhere....hopefully my ramblings make a little more sense now.