I'm curious as to what modules others would be using with their PRO3, especially if they intend on creating a permanent and fixed one, specifically for their PRO3.
With the PRO3's CV ins and outs and gate, in conjunction with using the DC offset mod matrix source, you should be able to set up some single modules to be completely controlled by your PRO3, and even saved with your preset that way... in that case you would of course need to have a module that do not have more ins and outs than 4/4 and a gate, unless you plan on only controlling some parameters and leaving the other in a static position. There are many oscillator modules that could be completely controlled by 4 ins and four outs and a gate signal.
Personally I have decided on this dual sampler Eurorack module (see picture) to be a steady companion with the PRO3's CV/Gate in and outs... Will give some nice abilities to mess around with one shot single and looped samples in the PRO3 engine... I don't want more modules because the mangling of this module is the PRO3's job via it's CV ins and outs... that way I can create presets on PRO3 that can be saved and recalled, used for samples, and the only parameter I need to do manually is the sample itself, which is exactly the only parameter I would not want to be saving at all, but rather be browsing manually from the module itself... thus this combi will become hardwired... a PRO3 with two analog oscillators, a Wavetable oscillator and two sample oscillators
The module has CV input 1V/O plus for changing sample start, length and even the sample itself, so I could in essence completely store it's whole setup in a PRO3 preset, simply by using mod source DC to select the sample. It also has input for reverse playback, and even an output that signal when a sample stop playing... changing all of these parameters and having the option to use PRO3 envelopes, LFOs, Sequencer etc. is quite nice... and all that even at audio rate.
The module also lets you sample, and even via a gate signal from the PRO3 if you want to.
Each SD card can store up to 600 samples set in banks with 10 samples each. quite powerful module.
Personally I'll never enter the rabbit hole of getting a large case and a number of modules, so I am only going for completely integrated single module setups... I know that you can mangle a sample a thousand cool ways with more modules, but I wanted all modulation to be PRO3 because this allow me to save many different mangling presets meant for samples without having to constantly setup a larger Eurorack system.