Good question! There’s a subtle underlying issue I need to address in order to get those working correctly:
https://github.com/stewdio/q.js/issues/18
I have an idea of how to solve it while still keeping gate creation simple—I just need to find some downtime to investigate further :)
Like, can I say negate index 1 insofar as both index 2 and index 3 are on?
I tried adding two identity things and an x thing, and selecting the 3 of them, but the "c" button wasn't enabled when I did that.
Uh, that is still unitary, right?
Ok, yeah, it still permutes the 8 pure states, and permutation matrices are orthogonal, and therefore unitary, matrices.
edit: Oh, I see, in the https://quantumjavascript.app/Q-Gate.html page it says that that gate (apparently called the "toffoli gate" or the CCNOT gate) is "coming soon".
So, it will be added, but hasn't been quite yet.