I'm curious...
Have you investigated a source + cost of the basic rail you might use in a "very large 3-rail O-scale multi-deck layout"???
Just wondering what that information might do to your enthusiasm for this approach....
One of my best friends in this hobby....Frank...insisted on hand-laying his entire spaghetti bowl, basement-consuming HO layout. And that over an unbelievable spline roadbed consisting of 1"X clear white pine stock ripped to 1/4" widths by the local lumberyard/millwork specialty, then splice-glued to a set of risers built onto a CWP egg-crate benchwork. The eleventy-seven bazillion custom switches worked flawlessly....from a bank of hand throws through cables. It was an incredible sight as it unfolded...over 35+ years time.
And then he died.
Frank was of the this-is-the-only-way-to-go crowd. But in the end, for him it was a hobby unto itself....bench work, roadbed....hand laying track. He never uttered a word of regret to me. He was proud of his accomplishments. He accumulated a bunch of already-built buildings, bridges, gizmos, and gadgets to 'someday' complete his layout. In truth, he had a quiet disdain for spending a lot of time on scenery. In fact, for those tour layouts that were spectacular for their scenery, Frank was always trying to look UNDER the layout, ask the creator(s) about the benchwork, roadbed, track, switch contstruction/control....which would lead into lengthy exposé about HIS approach. But he was truly an enthusiastic friend. I miss him.
Tearing down his layout was particularly sad. There wasn't anything...zero, zip, nada...of the trackwork that was salvageable or reusable. Even the benchwork was in pieces and perforations so numerous to be of no interest, free for the taking, among local craftspeople, hobbyists. We tried.
But, one thing I took away from Frank's lifetime experience was a re-balancing of priorities, approaches. I was mostly into HO when I shared time with him. Failing eyesight and dexterities... and a belly full of pickers-of-nit than seemed to dwell in that scale locally!...led me to O3R. Would I do what you propose, what Frank did??.......never entered my mind. I ultimately chose the Ross/Gargraves approach, working a layout plan through RR-Track and its libraries of components. I'm a long way from 'completion' of the layout (never really happens as many will tell you!), but I'm enjoying each challenging step......EXCEPT electrical gremlin-searching!!
But, that's what's great about the hobby.....something for each and every.
BTW, your interest brought to mind an old issue of Model Railroader magazine...from the 50's-60's of the last century, I recall. The cover story featured a gent in Britain who had hand-laid his entire O-scale layout to fine-scale British standards. The bull-head rail was supported on every tie with 'chairs' (mounted on cardstock ties) as in the prototype. The sweeping broad curves, faultless turnouts, nary a kink, wriggle, or glitch noticeable.....anywhere. The eye was drawn to the trackwork.....everywhere. Even at my youthful age I knew craftsmanship of trackwork when I saw it. Holy moly! THEN, I noticed the locomotives and rolling stock....nicely done, of course, but nothing particularly eye-catching. And THEN I checked the 'scenery'....a building, a swichtower, a station, here or there. (ho-hum) But, that hand-laid trackwork!?!?!?,,,,,,,,,absolutely incredible. You'd think I'd have saved that issue.....but, no. I came across it at a flea market once....equally drawn to the images even 40-50 years later!
So, if it's your 'thing', go for it!
And, best of luck......and share your efforts with others on the forum. You may start a whole new group of gandy-dancers!
KD