The Rebex Tiny Web Server can accept only .pfx, which contains (public) certificate and associated (private) key.
It seems you need to combine all the files into a .pfx file.
I suppose that the .crt file contains the required private key in PEM format. View the file, it should start with -----BEGIN PRIVATE KEY----- or -----BEGIN ENCRYPTED PRIVATE KEY-----.
The .p7b file should start with -----BEGIN PKCS7-----.
You need to first convert .p7b into PEM as described here:
openssl pkcs7 -print_certs -in cert.p7b -out cert.pem
The .pem file should start with -----BEGIN CERTIFICATE-----.
Then you can create .pfx file as described here:
openssl pkcs12 -export -out cert.pfx -inkey cert.crt -in cert.pem
Or check, whether your certificate vendor can provide .pfx certificate directly.