Add UNSET option to amrflags in 3d code

[rjleveque]

This was done in #224 in 1d and 2d, with a change in the calling sequence of flag2refine. The same modification should be made in 3d, both for consistency and because it speeds up the code if regions are used to force or forbid refinement and the flag2refine process is costly (e.g. adjoint flagging).