Welcome to the forums!
I don’t have much to add to what the others wrote about 「三人」 and 「~人」 that has already been identified as a counter for people (counters are words you use to count a specific set of things: long things, flat things, people etc). You’ll learn it in lv 3: ~人
Just wanted to add that it’s not such an uncommon thing. The more a word is common and the more readings it has (on’yomi and kun’yomi). You’ll learn others in the next levels. I’ve heard people here say that the first kanji are also the most hellish, because of the many readings…
がんばれ!
Small Advice
If you hear Japanese (tv, anime, people around…) readings become much easier, so if you don’t already, try to see something you like in Japanese.





