  We report about the search for short-term variability in the high-energy
gamma-ray energy band of three flat-spectrum radio quasars (3C 454.3, 3C 273,
PKS B1222+216), whose flux at E > 100 MeV exceeded the value of 10^-5 ph cm^-2
s^-1 for at least one day. Although, the statistics was not yet sufficient to
effectively measure the characteristic time scale, it allowed us to set tight
upper limits on the observed doubling time scale (< 2-3 hours) -- the smallest
measured to date at MeV energies --, which can constrain the size of the
gamma-ray emitting region. The results obtained in the present work favor the
hypothesis that gamma rays are generated inside the broad-line region.
