C3 is more comparable to Odin or the Better C mode in D in that it tries to be a pragmatic evolution not revolution of C.

Here is a comparison to Zig in terms of features: https://c3-lang.org/faq/compare-languages/#zig

And yes, they are all system programming languages with a similar level of abstraction that are suited for similar problem. It is good to have choice. It is like asking what do you need Ruby for when you have Python.