A "poor man's" 3-line bellcrank can be made out of two ordinary two-line ones. Fox steel ones would be great. Aluminum Perfect's can work, but I wouldn't put a "big" plane on them...may .19-.25 max.

Keep one stock - that's the elevator, then mount that one on a second one just to the rear of the pivot by maybe 1/2" or so, and chop off the remaining part of that arm.

Now what you have is an ordinary crank that will move a bit from inboard to outboard as the throttle line is pulled, but not enough to screw around with the controls enough to be noticeable .

What needs to be done next is mount a wedge/pin/bolt into the crank mount to limit the outboard motion of the throttle crank arm because all your 2-line tension will be trying to pull towards the inboard (making the throttle crank arm swing outboard mashing ribs and such. A wedge/pin/bolt limiting its swing to maybe 30 degrees or so seems to work OK. Most throttles won't need much movement, and if you started with 3" cranks, the arm the throttle rod goes on has a lot of swing already

I've built and used a few of these and they work quite well. Not as fancy as an old Walker style with arcing grooved pivots and such but cheap, effective, and durable.