At a width of 36" the MDF will sag substantially under its own weight.
If it can be fastened to a surface that will keep it from sagging, it
will be fine. I am not familiar with your application but it sounds
like the top can be fastened to the underside of the top of the bench
and the bottom will have to be blocked up or have a base that can keep
the MDF from sagging. If you use MDF shelves at this width, it would
be best to incorporate at least a built up front edging, with either a
rabbet to receive the shelf, or perhaps a bisuited connection between
the edging and the shelf material, and provide a point of support at
midspan along the back edge of the shelf.