I have a re-seller account and each account has the ability to install “Let’s Encrypt”.
Some of the domains use Google Apps and therefore are Google MX records.
Should I “Install mail SMTPS/POP3S/IMAPS SSL certificate for X.C0M” If the MX records are at/pointing to Google?
Google manages the SSL/TLS configuration for their servers automatically. You shouldn’t need to do anything unless you’re an enterprise managing your own mail servers in addition to Google Apps. All you need to do is make sure your MX records are configured according to https://support.google.com/a/answer/174125?hl=en.