I solved the level for Act 3 Spring, a.k.a. 3:6, of the Auditorium demo once I stopped trying to split the beam (or “flow”, in Auditorium parlance).
Have one large attractor (“attract control”) pull the entire beam through the yellow filter (“color sphere”) and then counterclockwise to the right through the lower yellow amplifier (“container”). Place the left arrow (“directional”) so that it pushes the beam as it enters the three o’clock position and make it large enough to keep pushing the beam more or less on a straight line leftward through the upper yellow amp.
If done properly, the beam will continue to the left, pass through the blue filter, and through the rightmost blue amp. Set the second attractor so that it takes this now blue beam and curls it clockwise through the other two blue amps. The beam will naturally pass through the red filter and trigger the red amp when you do this.
There’s actually one more level after 3:6, but it’s inconsequentially trivial; all it does is show off a rabbit button, which speeds up beams.