Hey Catman1...
Looking back through some of your photos, is there a pivoting bracket comming off the center 3ph link on the blade which your 3ph center link is then attached to?
If so, that pivoting link HAS to be for use when you are also using the guage wheel. Rather than completely removing the center link (as suggested in the manual), that pivoting bracket will let the tractor pitch up and down in front of the blade without pushing/pulling the blade in the opposite direction with the center link on the 3ph. This set up is also used on 3ph mounted rotary mowers to allow the mower to follow ground contours independtly from the up and down pitching of the tractor as it goes up and down through the various dips and hills while mowing.
I bet if you connected your center 3ph arm directly to the blade you'd get a LOT more lift capability.
Ah ha!
I just dug through the RBT45 manual again and indeed I'm seeing a diagram on page 13 that shows a part called a "center floating link". I am reasonably certain you only want to use this if you are using the gauge wheel. Remember that you do not HAVE to use the guage wheel all the time. Actually, that guage wheel is only needed when you are trying to act like a road grader doing leveling work.
Bottom line is that I bet if you remove that center floating link and connect your center 3ph link directly to the blade (as implied by other diagrams which do NOT include the guage wheel), you should be able to lift the blade at least 3' high.
Give it a shot...