1998 jimmy blazer 4wd-a/c

What would prevent the a/c button to stay in the on position, when try to
turn on the a/c the light flashes 3x then stays off. I recently had a new
compressor, dryer,& tube chg and flush out the system. All the fuses are
good, but the a/c won't turn on.
And just a couple of days ago, hot air started coming thru the vent when
the blower is in off position. Heat is coming from some were because the
panel get very hot. I don't know if this has anything to do with the a/c
or if I just have to replace the thermostat. Any suggestion will help.