  A domino is a $2\times 1\times 1$ parallelepiped formed by the union of two
unit cubes and a slab is a $2\times 2\times 1$ parallelepiped formed by the
union of four unit cubes. We are interested in tiling regions formed by the
finite union of unit cubes. We investigate domino tilings, slab tilings and
mixed tilings (vertical dominoes and horizontal slabs). A flip is a local move:
in a slab (resp. domino) tiling, two neighboring parallel slabs (resp.
dominoes) are removed and placed back in a different position.
  We may interpret a three dimensional region as being made of floors. A slab
or domino which crosses two floors is vertical, and horizontal otherwise. We
can also define a flip in a mixed tiling, we replace four vertical dominoes in
a $2\times 2\times 2$ box by two horizontal slabs and vice-versa.
  Inspired by the twist for domino tilings we construct a flip invariant for
mixed tilings. Based on this, we then construct the triple twist for slab
tilings: an invariant under flips assuming values in $\mathbb{Z}^3$. We show
that if the region is a large box then the triple twist assumes a large number
of possible values, at least proportional to the fourth power of the volume. We
also give examples of smaller regions for which the triple twist assumes only
one value and the set of tilings is connected under flips.
