a simple question about jit.gen with 2 and 3-dimensional matrices

I am experimenting with [jit.gen] to control a matrix. I understand that [snorm] will give me normalized values from -1 to 1 across all incoming dimensions. I can split these up with [swiz] and [vec], so as to alter the specific values of whichever axis I'm working with.

However, if I wanted to input a 2d matrix (like from [jit. noise], for instance) and alter the z input of [vec], based on something like pixel brightness. How would I go about deriving a single value from a 2d matrix and using it as the third element in a 3d matrix?

My apologies if this question doesn't make sense - I am struggling to understand something here, also.