  We present new observations of the transiting system TrES-3 obtained from
2009 to 2011 at several observatories. The orbital parameters of the system
were re- determined and a new linear ephemeris was calculated. The best quality
light curve was used for light curve analysis, an other datasets were used to
determine mid-transit times, Tc, and study transit time variation (TTV). For
planet parameter determination we used two independent codes and finally, we
concluded that our parameters are in agreement with previous studies. Based on
our observations, we determined 14 mid-transit times. Together with published
Tc we found that the timing residuals showed no significant deviation from the
linear ephemeris. We concluded that a periodic TTV signal with an amplitude
greater than 1 minute over a 4-year time span seems to be unlikely. Our
analysis of an upper mass limit allows us to exclude an additional Earth-mass
planet close to inner 3:1, 2:1, and 5:3 and outer 3:5, 1:2, and 1:3 mean-motion
resonances. Using the long-term integration and applying the method of maximum
eccentricity, the region from 0.015 au to 0.05 au was found unstable and the
region beyond the 0.05 au was found to have a chaotic behaviour and its
depletion increases with increasing values of the initial eccentricity as well
as inclination.
