  We introduce a purely geometric formulation for two different measures
addressed to quantify the entanglement between different parts of a tripartite
qubit system. Our approach considers the entanglement-polytope defined by the
smallest eigenvalues of the reduced density matrices of the qubit-components.
The measures identify global and genuine entanglement, and are respectively
associated with the projection and rejection of a given point of the polytope
on the corresponding biseparable segments. Solving the so called `inverse
problem', we also discuss a way to force the system to behave in a particular
form, which opens the possibility of controlling and manipulating entanglement
for practical purposes.
