Is S2/S3 of Uma Musume required for Cinderella Gray?

Each season of Uma musume: Pretty Derby tell the history of different horses in different era. So technically speaking, you can start Cinderella Gray right away without any knowledge of uma musume

No not required, each season focuses on different horses and their own rivals. But if you want to know who some of the supporting characters are and their relationships (world-building stuff) then you can watch the previous seasons. I highly recommend the latest movie though (Uma Musume: Pretty Derby - Shin Jidai no Tobira), the animation and production quality in that is definitely top-notch.

Cinderella Gray is set in the timeline before any of the other seasons, in the anime timeline of Uma Musume Oguri has already retired from being an active racehorse in Season 1. So no, you don't. (though you should because they are good)

Should you do it? Yes
Do you need to do it? Not really

Uma Musume follows different horses in different time periods and Cindrella Gray in particular starts before the main series so, you should be fine starting from here but, I do recommend watching the anime firsthand as it is a better experience that way, regardless.

Cinderella Gray is a fully standalone series and you could even argue it taking place before all of the other uma stuff if you look at the irl racing timeline

Be for real.