There are third party tools to do just that. I usually use OpenSSL tool that comes with OpenVPN, as I use OpenVPN quite frequently on multiple machines.
If you have it installed, you can try to run something like:
openssl pkcs7 -print_certs -in certificate.p7b -out certificate.cer
Alternatively, in Windows you can usually also just double-click the .p7b file - a window will appear with the contents of the .p7b container, where you can browse through the certificates contained within and export each one individually to a separate file in a format you select. Just double-click the certificate and click Copy to file in the Details tab.