Binocs are legal at 3d shoots. Rangefinders are not legal at 3d shoots.
But, it seems pretty easy to use normal binoculars as range finders...
Start at practice range and get binoculars focused well on 30 yard target. Learn which way to turn focus wheel to focus on 40 yard target.
At the first real target, look through the binocs. Is target perfectly focused? If so, target is exactly 30 yards. If not, which way to turn the knob to get it focused? Then you know whether it is more than 30yards or less than 30 yards.
This seems pretty easy. Other folks have to already figured this out.
Do you consider the following scenario cheating? And would your answer depend on whether it was done in a large tournament or a small local archery club? An archer competing in a 3D shoot walks up to the first shooting stake and using a set of high quality binoculars glasses the target to determine an aiming point for the 11 ring. The archer adjusts the binoculars to have the sharpest image possible and after judging the yardage at 40 yards executes the shot perfectly and scores an 11. The archer proceeds to the next station and at first glance thinks the target is a little closer than the last shot, maybe 37 – 38 yards. The archer then glasses the target but notices’ adjusting the knob slightly counterclockwise is required to achieve the same sharp image as the previous target. Almost subconsciously from years of experience the archer knows if the binoculars have to be adjusted in this manner from the last shot that was 40 yards, then the target has to be a little farther, not closer and rethinks and adjusts the judged yardage thus making a better shot and achieves a high score. If binoculars were not used and the shot was made on the first judgment, the score would have been less. Well, do you consider this scenario cheating? Is your answer influenced by the size of the shoot or the organization hosting it? Is this a good reason to make binoculars illegal at 3D shoots?
Those are already available.
But it seems that you could get most of the benefit by just knowing which way to turn the focus wheel to bring the image into focus.
In the book Idiot Proof Archery, Bernie Pellerite covers in Chapter 15- "3-D and Estimating Yardage- Legal and Illegal" pp.271-290 this subject. It is an interesting read.