The designation 3I means the third interstellar object discovered. 1I (Oumuamua) was discovered in 2017, and the new Vera Rubin telescope is likely to discover a lot more in the future.
We're just at the beginning of finding this new population of objects, and I think it's very exciting. It's like when Ceres was first discovered (in 1801) and suddenly people found a whole population of asteroids between Mars and Jupiter. We're in a similar time now with interstellar objects.
How many interstellar objects pass through the Solar System each year? How many will we find in the next 10 years? We're about to find out.