I am trying to create a certificate for SHA-3. Which tool can I use to create it?
Currently, I am using SHA-256. Will OpenSSL work in this case? If yes then how?
This is generally not considered a good idea. The idea of using a certificate is to allow other people to verify the identity of whatever you are serving. SHA-3 is still relatively new and support for legacy devices is a problem. This Answer explains more about that in detail.
Further, the post you linked explains how to use a SHA-256 hash for your certificate. SHA-256 belongs to the SHA-2 family, not the SHA-3 family.