As the author, let me add something beyond the comparison. Zig and Odin are very different languages, Odin is – as its slogan goes - "for the Joy of Programming". Zig on the other hand doesn't feel that this is a goal. From what I can tell Zig fans like to wrestle with the features of Zig to figure out how to fit their solutions within the constraints of the language. A mental challenge, similar to that of fighting the borrow checker in Rust. People who "figured out" Zig tend to be fiercely loyal to the language in a similar way as Rust evangelists to Rust.
C3 has a lot in common with Odin, but very little in common with Zig.
C3 has a slightly different feature set than Odin (e.g. more compile time execution, has methods, contracts, but doesn't have Odin's matrix programming and more extensive runtime reflection), but the goals aligns strongly with Odin's.
If you prefer C syntax and semantics you might like C3 better, but Odin is a fine language too.