Zonda’s were updated last year. Racing 3’s are now a 2018 model I believe. This usually has an affect in discount levels, however there appears to be a £110 difference in RRP figures if you believe those.
Without actually seeing them side by side, it’s not possible to compare the rims etc.
Personally, I prefer the spoke patterns on the Campag wheels.
I do wonder whether they are priced lower as some people just assume Campag wheels are only for Campag groupset bikes?
I have had Zonda’s myself and now using Eurus, after the former were damaged. The Eurus are even better than the Zonda as they are stiffer, but then you are paying more for that privilege obviously.