I saw someone mentioned that if something happened and a seat was vacated early, one of the justices whose term had expired could be brought in to serve in that seat until it is filled with a new person at the next scheduled time. But that seems to mean that some other justice(s) more senior than the one who vacated might wind up serving more than the normal 18 years as they wouldn’t be replaced until the next cycle.
As far as a president getting to appoint 3 justices in one term, we just had that with trump. IMO there really should be more than just 9 seats. A common suggestion is 13, one for each District.