How to bind a domain to a subdomain on Apache?

Question

Apache HTTP Server

Hello!
What settings need to be done to link domains to subdomains?
Let me explain the situation: I have 3 subdomains on 1 main domain.
I want to assign my own domain to each subdomain, and for this I set up CNAME on each of the 3 domains for my subdomains.
Everything works, but the domains do not refer to a subdomain, but to the main domain.
It can be very confusingly explained, but in fact everything is simple. It seems to me that something needs to be added to Apache so that everything starts working. Does anyone know how to solve this problem?