The three of 'em are essentially the same age - played at 29 in 2019, will play at 30 in 2020. All three of them are showing decline from their peak OPS+ seasons, which occurred in each case in the age range of 26-28 (surprise, surprise). Discounting Gregorius' 2019 stats a bit, because of the injury, it looks one could expect similar OBP from each of them in 2020 (around .325 or so - not bad, but not special either). Gregorius has a bit more power than the other two. Defensively - Cesar is deficient anywhere but 2B; Segura and Gregorius can play a respectable SS, and presumably 3B.
It almost doesn't matter, as best I can tell, which two of these three man the two middle infield positions. If you add a third one (3B), you're taking PA from somebody else. That's going to be either Kingery or Haseley...both younger players who likely have not reached their offensive peaks.
From my perspective, signing Gregorius and moving Hernandez, or moving Segura, really doesn't change much (except the names on the jerseys - might sell a few more of them). Having all three on the roster might actually be a negative, if Kingery or Haseley loses PA (which might be more productive PA, and might contribute to offensive development/growth for either of them).
Now, if Gregorius at 30 rebounded to his 2018 performance (OBP and SLG), that would be an improvement - but my guess is that, while he'll likely be better than his injury-shortened 2019, he won't return to the 2018 level - it's the age thing again. 30-year-old players are starting down the back side of the hill.
I wouldn't mind their signing Didi... but I don't expect it to make a lot of difference. Running in place, waiting for Bohm (and counting on Bohm being the real thing).
Love to be wrong, though!