Something like this:

  use bitvec::prelude::*;  // BitView trait, Msb0, etc

  let my_input = vec![0b11100000u8];
  let bitslice = my_input.view_bits::<Msb0>();
  
  let my_num = bitslice[..3].load_be::<u8>();

  // continue parsing after my_num
  let nextslice = &bitslice[3..];

You would use Lsb0 instead of Msb0 if you wanted to parse bytes starting at the low bit.

It would be more convenient with a peek/take style wrapper over the slice, but I don't think one is included in the crate.