While we have no idea when in 2021 the movie will be released, given The Kissing Booth 2 had a July release date, it makes sense the third movie will come out a year later.
The Kissing Booth is based on a book from British authour Beth Reekles, who has, at present, only written two of the books.
As a result, it is hard to tell what the third movie will be about, and who has been involved in writing it.
However, Reekles announced on her Twitter that she had been keeping this a secret, implying she has been just as involved.
source: express.co.uk