I just watched video 156. I see what you mean.
The purpose of the video is to show how using an instrument object allows you to change the levels of each channel independently. However, by moving the Instrument object's level, all channels cabled to it will adjust to the precise level (and not maintain their relative levels).
The best way around this is either:
- change each channel strip independently or
- open the Mixer or Environment, select the 3 channel strips and adjust one of the level faders.
Another way would be to use a SI channel strip instead of the Instrument object & insert a gain plugin on the "Controlling" CS and adjust the level in the gain plugin.
Alternatively, (yes, there are probably many more variations on this method!) In I/O of the 3 channel strips, change "Output 1-2" to an empty Bus.
A new Aux channel strip is created (you'll see it in the Mixer) using the input of the Bus you chose. Now you can use that to control the overall level of the channels...
Hope that helps :)