The regular rules for Aliyah positions are based on three rules: 1) kohen and levi read first if available to avoid fighting about who reads first (Gittin 59a), 2) a kohen should not read directly after a different kohen, nor levi after another levi, so as not to make people wonder if the first one was found to be invalid (Gittin 59b), 3) some say Rule 2 applies even if the two are not directly adjacent since the latter 5 aliyot are generally for Yisraelim some might come to question the validity of the latter kohen/levi (cf. Shulchan Aruch OC 135:10), and some say not to worry about this third rule if calling another Kohen-Levi-Yisrael triad (cf. Mishna Berura, ibid :37).
The Shulchan Aruch (OC 135:12) rules that if you're stuck then we allow breaking all those rules. Since Rule 3 is highly disputed, that'd be the first to go. Accordingly, it comes out that you basically do as close to a repeating Kohen-Levi-Yisrael as possible as many times as needed to fill out 7 people.
There are two caveats to keep in mind. First, if the case is one with at least one Yisrael and no Levi, then the first triplet would be Kohen-Kohen-Yisrael like an ordinary situation of no Levi, but not subsequent triplets since those are arguably not actually necessary as triplets. Second, if there are only Kohanim and exactly one other, or only Leviyim and exactly one Yisrael, the Shulchan Aruch (:12) rules we flip the script and put the singleton first to avoid fighting and continue with repeating statuses from there.
Example case of 2 Leviyim and 1 Yisrael: Kohen, Levi, Yisrael, Kohen, Levi, Kohen, Kohen.
Example case of 0 Leviyim and 3 Yisraelim: Kohen, same Kohen, Yisrael, Kohen, Yisrael, Kohen, Yisrael.
Example case of 0 Leviyim and 1 Yisrael: Yisrael, Kohen, Kohen, Kohen, Kohen, Kohen, Kohen.