  We propose a semianalytical method to compute the strengths on each of the
three massive bodies participating in a three body mean motion resonance (3BR).
  Applying this method we explore the dependence of the strength on the masses,
the orbital parameters and the order of the resonance and we compare with
previous studies. We confirm that for low eccentricity low inclination orbits
zero order resonances are the strongest ones; but for excited orbits higher
order 3BRs become also dynamically relevant. By means of numerical integrations
and the construction of dynamical maps we check some of the predictions of the
method. We numerically explore the possibility of a planetary system to be
trapped in a 3BR due to a migrating scenario. Our results suggest that capture
in a chain of two body resonances is more probable than a capture in a pure
3BR. When a system is locked in a 3BR and one of the planets is forced to
migrate the other two can react migrating in different directions. We exemplify
studying the case of the Galilean satellites where we show the relevance of the
different resonances acting on the three innermost satellites.
