  We present Warm Spitzer/IRAC secondary eclipse time series photometry of
three short-period transiting exoplanets, HAT-P-3b, HAT-P-4b and HAT-P-12b, in
both the available 3.6 and 4.5 micron bands. HAT-P-3b and HAT-P-4b are
Jupiter-mass, objects orbiting an early K and an early G dwarf stars,
respectively. For HAT-P-3b we find eclipse depths of 0.112%+0.015%-0.030% (3.6
micron) and 0.094%+0.016%-0.009% (4.5 micron). The HAT-P-4b values are
0.142%+0.014%-0.016% (3.6 micron) and 0.122%+0.012%-0.014% (4.5micron). The two
planets' photometry is consistent with inefficient heat redistribution from
their day to night sides (and low albedos), but it is inconclusive about
possible temperature inversions in their atmospheres. HAT-P-12b is a
Saturn-mass planet and is one of the coolest planets ever observed during
secondary eclipse, along with hot Neptune GJ 436b and hot Saturn WASP-29b. We
are able to place 3$\sigma$ upper limits on the secondary eclipse depth of
HAT-P-12b in both wavelengths: < 0.042% (3.6 micron) and <0.085% (4.5 micron).
We discuss these results in the context of the {\it Spitzer} secondary eclipse
measurements of GJ 436b and WASP-29b. It is possible that we do not detect the
eclipses of HAT-P-12b due to high eccentricity, but find that weak planetary
emission in these wavelengths is a more likely explanation. We place 3 sigma
upper limits on the |e cos(omega)| quantity (where e is eccentricity and omega
is the argument of periapsis) for HAT-P-3b (<0.0081) and HAT-P-4b (<0.0042),
based on the secondary eclipse timings.
