How to install Multiple Intermediate CA Certificates & Create Certificate Bundle

cat COMODORSADomainValidationSecureServerCA.crt COMODORSAAddTrustCA.crt AddTrustExternalCARoot.crt > PositiveSSL.ca-bundle

This will create a file named PositiveSSL.ca-bundle containing the 3 CA (Certificate Authority) intermediate certificates, all in the proper order. Each CA certificate basically validates the one next to it, top to bottom, from the root CA certificate that your Browser has on file, through the intermediate

Update March 2018.

cat COMODORSADomainValidationSecureServerCA.crt COMODORSAAddTrustCA.crt AddTrustExternalCARoot.crt > Comodo.ca-bundle

certs, and down to the public certificate file.

Configure the website’s SSL VirtualHost file to use the private key, public certificate, and the bundled intermediate CA certificates chain file.

Apply these, restart Apache.

Creds: http://www.devside.net/wamp-server/installing-comodo-positivessl-certificate-bundled-with-root-and-intermediate-ca-certificates-on-apache