The need for multiple transits is indeed to make sure it's a planet. This is a rule followed by the discoverers to make sure that they don't have to make a retraction later.
So let's say you discover a transit. You wait patiently for another transit, and it comes four days later. Yippie! You found a planet. But then four days afterward, you don't get a transit. Then two days after you expect it, you get another transit. Continued observations reveal that you've found two transiting planets in the same system, and what you thought was the orbital period of a single planet turns out to be the time inbetween transits of the two planets. The time inbetween transits may change if your photometric precision is low enough that sun spots trick you into thinking you found a transit. Since sun spots move around, you'll get transits that occur with changing regularity. Thus multiple transits are needed to check for consistency.
First transit -> You discovered a transiting object.
Second transit -> You have a lower limit on its orbital period.
Third transit -> Pattern repeats, orbital period confirmed.
This isn't IAU rules, and the IAU has nothing to do with confirming or disproving exoplanets. They're not even rules, but rather guidelines that anyone would want to use to make sure that they don't have to retract something later.
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