  Chiral symmetry breaking may exhibit significantly different patterns in two
chiral limits: N_f=2 massless flavours (m_u=m_d=0, m_s physical) and N_f=3
massless flavours (m_u=m_d=0=m_s=0). Such a difference may arise due to vacuum
fluctuations of s-bar{s} pairs related to the violation of the Zweig rule in
the scalar sector, and could yield a numerical competition between
contributions counted as leading order and next-to-leading in the chiral
expansions of observables. We recall and extend Resummed Chiral Perturbation
Theory (ReChPT), a framework that we introduced previously to deal with such
instabilities: it requires a more careful definition of the relevant
observables and their one-loop chiral expansions. We analyse the amplitudes for
low-energy pi-pi and pi-K scatterings within ReChPT, which we match in
subthreshold regions with dispersive representations obtained from the
solutions Roy and Roy-Steiner equations. Using a frequentist approach, we
constrain the quark mass ratio as well as the quark condensate and the
pseudoscalar decay constant in the N_f=3 chiral limit. The results mildly
favour significant contributions of vacuum fluctuations suppressing the N_f=3
quark condensate compared to its N_f=2 counterpart.
