My interpretation is that your cohort would be 1st level and you would have followers as normal. If you were lower than 3rd when you gain your first tier, then I would say either A) No cohort till at least 3rd level or B) Your cohort is 1st level but doesn't gain any xp until you hit 3rd, then progresses as normal.
Calculate your leadership score as normal.
Then follow the normal leadership rules for cohort level
Cohort Level: You can attract a cohort of up to this level. Regardless of your Leadership score, you can only recruit a cohort who is two or more levels lower than yourself.
So at 3rd level, the highest level cohort you can recruit is 1st.
Number of followers is also calculated normally.