  We present 22 new transit observations of the exoplanets WASP-18Ab, WASP-19b,
and WASP-77Ab, from the Transit Monitoring in the South (TraMoS) project. We
simultaneously model our newly collected transit light curves with archival
photometry and radial velocity data, to obtain refined physical and orbital
parameters. We include TESS light curves of the three exoplanets to perform an
extended analysis of the variations in their transit mid-time (TTV) and to
refine their planetary orbital ephemeris. We did not find significant $\rm
TTV_{RMS}$ variations larger than 47, 65, and 86 seconds for WASP-18Ab,
WASP-19b, and WASP-77Ab, respectively. Dynamical simulations were carried out
to constrain the masses of a possible perturber. The observed RMS could be
produced by a perturber body with an upper limit mass of 9, 2.5, 11 and
$4~M_{\oplus}$ in 1:2, 1:3, 2:1, and 3:1 resonances in the WASP-18Ab system. In
the case of WASP-19b, companions with masses up to 0.26, 0.65, 1 and
$2.8~M_{\oplus}$, in 1:2, 2:1, 3:1, and 5:3 resonances, produce the RMS. And
for the WASP-77Ab system, a planet with mass between $1.5-9~M_{\oplus}$ in 1:2,
1:3, 2:1, 2:3, 3:1, 3:5, 5:3 resonances. Comparing our results with RV
variations, we discard massive companions with $350~M_{\oplus}$ in 17:5
resonance for WASP-18Ab, $95~M_{\oplus}$ in 4:1 resonance for WASP-19b and
$105~M_{\oplus}$ in 5:2 resonance for WASP-77Ab. Finally, using a Lomb-Scargle
period search we find no evidence of a periodic trend on our TTV data for the
three exoplanets.
