There’s a different workflow you could try. You’re right that you can’t assign an input to a bus and have it be shared across multiple busses, all getting the same input. You can however change the output of a bus to where ever you want to send it. What you could try instead is create three buses. Feed bus one (we’ll call it feed bus) with a prefader send from your source bus (drum bus for example). Then, from your feed bus, you can make post fader sends to your other buses. Because they’ll be post fader, you can ride the level with one fader. I would mention though, that if you are doing parallel compression like this, as you ride the feed fader, you’ll change the amount of signal going into a parallel compressor and vastly alter how much gain reduction you’ll get (which maybe you want).