I have three siblings.
There were only two or three people in the river.
Out of those three people, which one is your type?
You have the kanji for three and person. Common sense will tell you what this word is. It's three people.
This jukugo (combo kanji) word uses the on'yomi readings that you learned when you learned the kanji. That means you should know the reading as long as you know the kanji.