Is there an option to calculate SHA3-256 hashes in .NET Core? The goal is to recreate the isChecksumAddress util function in web3.js
No, there is no way of doing that now with pure dotnet core.
It's been on the watch list since it was announced. Since we don't implement cryptographic algorithms within .NET we're waiting on support from the underlying platforms (Windows CNG, Apple Security.framework, and OpenSSL).
See this issue.
But you might have a better luck with BouncyCastle. It has an implementation here but I don't know if it is out yet (in nuget).