The new Readiness score and higher-frequency HRV measurements require the new Health Sensing System in Apple Watch Series 12 and Apple Watch Ultra 4. So those particular features won’t come to the Ultra 3 just through a watchOS update. (Apple¹)
Health Age is a little different. Apple says it requires an Apple Watch, but doesn’t list it as exclusive to the new Series 12 or Ultra 4 hardware. It’s part of the redesigned Health app and uses data such as VO2 max, resting heart rate, sleep, and HRV. (Apple¹)
So with an Ultra 3, I’d expect Health Age, but not the new Readiness score or the new higher-frequency HRV sensing.
[1]: Apple advances health and fitness capabilities using Apple Intelligence - Apple