Matching the center channel matters indeed, no questions about it. I guess your doubt is more likely to know how detrimental can it be to not match your center channel.
You can always use a different brand or series for the center channel and EQ them to have a similar or even exact target curve as your mains and get away with murder, it will sound nice and most people wouldn’t know with their eyes closed and just enjoy it. I’ve done this in the past and the results are just like logic would dictate: you can do it, it's not a crime, but of course, it won’t be as good as a great matching center.
After my musings with A brand mains and B brand center…there was something that never convinced me about how voices sounded even the room was fully treated and the center Eq with enough parametric filters to my desired target…I'm sure 99% of my HT attendees would have never complained but Reference accuracy was my goal…the issue with my center wasn’t the mismatch with the mains, it was the inherent design of being MTM (nested array with elevated tweeter, which should be better than standard).
That’s when I upgraded to one of the best centers of that time (circa 2014/15), a KEF R600c coaxial design. Not only did I benefit from an almost perfect In-Room smooth Olive-Toole curve tracking, but It was also best in class vertical+horizontal directivity, moving to different seats didn’t change the overall tonal balance which is awesome. This forced me to later EQ every remaining speaker from the system to mimic the R600c response, which lead me to the next level of performance.
Now I’ve got R3 mains and R2C in a different environment, and I can assure you I can’t complain about a single finch for its performance that both subjectively and objectively. That the R2C didn’t get a high preference score doesn’t mean a thing (at the same time it didn’t score poorly). You can look at the measurements and interpret yourself, the R2C measures pretty flat and smooth, with great directivity glove plots on any axis, and low distortion, I can’t get what else we could ask for a center speaker…Erin’s measurements indeed show a little dip in the midrange, which isn’t that gross nor broad, peaks are much worse from an audibility threshold perspective.
I haven’t been able to reproduce that dip on any of my several In-Room measurements (I even uploaded vertical vs horizontal response of R2C on R3 review). For the 799€ I got them brand new in white gloss, even for the double of the price I couldn’t think of something better that ticks the boxes “coaxial” and "stylish" for my salon, it's also part of the decoration.
To summarize: yes you can use a different center and eq them and will still enjoy your system. But matching your R3’s with a third unit or an R2C or KHT 3001SEc will be magnitudes better (both objectively and by pure logic), and will do justice to your R3’s. Pairing them with any non-coaxial center design would be a waste of mains and logically directivity mismatch.