You can absolutely use DVCO machines as a three voice synth.
I am working on a little project doing that right now while I want for a new synth.
I havent used an A4 but I will say I think it would be much more functional as a synth as that is what it is designed for.
DVCO has a fairly limited palette. It can sound nice but it takes digging, and then you only have 1 lfo (admittedly per voice which is kinda cool).
It is also somewhat of a pain to do sound design for, you need to use an external controller because copying settings across the three voices is not feasible. I mean you can do it but it gets tiresome pretty quick. I use an EC4.
Also you have to make a choice re fx and the other voices, like if you want a cool sounding synth sound drenched in reverb and delay you have to compromise for the other rhythmic tracks, or take the individual outs for the voices and process them independently. which again, totally doable, but a bit of a pain.
Control over voice allocation is difficult. I havent used the RK Cable but I have used an mpe controller, i just configure it to send midi over channels 1-3 and it works great (also polyphonic aftertouch woo).
I also use it with ableton and an m4l device to route midi to the channels in a round robin way, it works but the MPE note per midi channel works better for playing imo.
https://www.youtube.com/watch?v=Q_zfs-PEBkQ
In that I have stuff like filter cutoff for the 3 voices mapped to a perf, and controlling the perfs via midi cc through an ios app. I recorded the notes directly into the rytm sequencer from my mpe controller.
I have some more stuff I will record soon, less kinda plucky/stabby like the above. With a bit of reverb and chorus it sounds great imo. Then again that is true of most synths so you do have to ask if it is really worth the effort faffing around.
It’s a lot of fun and you can get some nice sounds but personally if i were planning on doing a bunch of music id be better off using something designed to be used as a synth.