There is a person who has an infectious disease. Suppose he infects x people in each round and how many people are infected after three rounds. Find formulas and explanations, mathematics, and a quadratic equation. Ask! Did the person infected by him infect others?

If the person infected by him doesn't infect others, he will be 3X+ himself after three rounds.

If the person infected by him also infects others, then x+ 1 after one round and (x+1) * x+1= (x+1) after two rounds?

What is (x+ 1) after three rounds? * x + (x+ 1)？ = (x+ 1)？