To be honest - I never heard of a sampler/sample plaver with CV in. They all have to be controlled via MIDI (unless they have a built-in keyboard, of course).
So use the MIDI out instead of the CV/Gate outs. There are three possible ways if you want to connect more than one sampler or, in general, instrument:
a) Use samplers/instruments that provide MIDI Thru so you can daisy chain them.
b) Use a MIDI Thru box such as the Kenton MIDI Thru or similar. They belong in every synth household anyway, at least if you use MIDI at all 
.
c) If you don't use more than two units, a simple splitter will work. It may not be the most professional solution, but it's cheap and does its job (I sequence my Volca Bass and Volca Beats this way, and that works absolutely flawlessly).
Beat me to it. I've had the below sitting in a reply box for two days now, lol.
Samplers generally don't do analogue controls. CV/GATE/PITCH are pre-MIDI analogue signals and cost a small fortune compared to digital processors which run the MPX 8's. I'm currently working towards a voltage controlled sampler but am need to do it via the modular world and it's going to cost at least 700aud for the sampling brain alone (1200 if I get the one my heart is set on) and then there are the CV and gate sources to feed the brain.
MIDI is what has run samplers pretty much exclusively since 1983. It works really well and early samplers even used MIDI to transfer samples from sysex fileplayers.
Now my question is, why 3 MPX8's? From what I gather they would make a great little drum bank, but you'd do better to buy one MPX16 and then say a Microgranny 2.5 and another little cheapy-synthy-thingy like a Roland SE-02 or a Korg Volca of some flavour. Don't get me wrong, I've considered an MPX8/16 myself as a drum bank and still suggest to those who want a quick sample playback machine one of those would do brilliantly, but they are what you pay for. The 16 has a larger sample memory and the performance pattern maker which is why I suggest that over the 8.
If you are ready to dive in the eurorack world, these might fit !
https://1010music.com/product/bitbox-eurorack-module-with-touchscreen
https://www.modulargrid.net/e/qu-bit-electronix-wave
Otherwise the suggestions below are good and might be cheaper 
Umm, modular isn't really an alternative here. First you are paying twice as much for something that is not a complete unit, as such. The sampler module is only one component of the whole; at least an envelope and a VCA are two more modules that are needed before the sampler is actually a sampler. Without them you will have a loud and untamed sample with all the hiss and other operating noise coming out of the unit when there are no samples being played. A simple little Akai sample playback machine has all that covered for under $200.
