Mapping the 3 custom controls so that they are all controlled by a single (additional) custom control should be pretty straightforward. You could just create the new custom control (let’s call it “master” for sake of discussion). Then go into the subchain for each of the existing controls, and set their modulation source to be the local control “master”. You could assign a gain amount (pos or negative), and also a +/- bias amount so that they could even move by different amounts and in different directions when “master” changes. Alternatively, you could use an external offset coming in through one of the ABCDx ports to simultaneously control all 3.
It sounds like what you actually want to do is to have something more like preset values. I.e. if “master” is at 1.0, then control 1 should “jump” to a specific value, control 2 should “jump” to a different specific value. If master is at 2.0 then they “jump” to arbitrary (but preset) new values.
I think that the latter is possible strictly using the ER-301. You would have to use recorded samples to store values for the destination controls for each preset stop, and slice the sample so that the “master” control could play the correct slice of the sample corresponding to the preset.
I will offer up this tutorial video I made a while back. It is not quite exactly what you’re trying to do, but it leverages the sample player as a means to store and recall CV values. It is probably enough for you to make the connection.
http://wiki.orthogonaldevices.com/index.php/ER-301/Joe's_Tutorials_and_Patches#Quantize_to_Any_Scale_with_Probability
Of final note, the monome teletype, using the i2c connection to the ER-301 would make something like this pretty trivial. So might mutable instruments frames.
The other thing you might want to have a look at is the somewhat tentative roadmap for the 301. (Also read the caveats that the list is not a guarantee).
http://wiki.orthogonaldevices.com/index.php/ER-301/Tracker
But some things that are pretty involved workarounds today may be trivially easy in tomorrow’s firmware. In your particular question I am looking at some of the plans in the “Unit Parameters” section.
One more edit: something like this might already be pretty easy if you were to build it in the middle layer (LUA code). There I think you might be able to define specific values that each control is allowed to stop at. I’m not 100% sure as I’m still learning the middle layer myself. And as you just got your 301 it might not be something you even want to think about yet, but thought I’d include it.