I see no reason why Kemp can’t appoint someone as soon as Perdue’s current term expires let alone 3 days later.
There will be a vacancy starting at noon on January 3. Of course Kemp can appoint a senator at such time, even though he only would serve for a week or two (the run-off is on Jan. 5, but it will take a few days to count all of the votes and certify the winner). Why should GA have half of its allotted representation for that time period?
Obviously, Kemp shouldn’t name anyone who could hurt Perdue’s or Loeffler’s chances in the run-off. No cronies, and no one controversial. And not Perdue himself, because it’s bad optics two days before the run-off (”look, that Kemp already called Perdue the winner—let’s show him!”). That’s why I think that the ideal person would be retired U.S. senator Johnny Isakson, health permitting.
BTW, if Perdue wins the run-off (as I assume that he will), the temporary senator can resign even before the certification and Kemp can appoint Perdue to start early. And if the appointee does serve for only three days, it would not be as short a U.S. Senate term as for this piece of work that the Democrat governor of GA appointed to serve for a single day back in 1922: https://en.wikipedia.org/wiki/Rebecca_Latimer_Felton