Final answer:
To limit telnet access to three specific users, you would configure user authentication settings and possibly use firewall rules to restrict access. It's advisable to use SSH instead of telnet due to security concerns.
Step-by-step explanation:
To limit telnet access to three specific users, you would need to configure user authentication settings on the server that is hosting the telnet service. One approach is to modify the /etc/securetty file (on Unix-like systems) to restrict login to those three users. However, for a stronger security measure, consider configuring user-based access control in the telnet server configurations or utilize a more secure protocol like SSH which inherently supports per-user access configurations. It is also highly recommended to use firewall rules to restrict incoming connections to the specific IP addresses for added security. Telnet is considered insecure because it transmits data, including passwords, in plaintext. Therefore, it's advisable to switch to SSH where possible.