Will the KA3 film even come out?
Well, the KA3 comics are shaping up just fine:http://marvel.wikia.com/Kick-Ass_3_Vol_1_7
They would have to wait until the KA3 comics to end before even deciding on making a third film, if any.
The KA2 comics ended around March 21, 2012. The KA3 comics should end around March 2014 as well, provided that there are no delays.
The KA2 film sequel was announced May, 2012.
So based on the above dates, we can be fairly sure that if there is ever going to be a KA3 film sequel, it would be announced around May/June 2014 if the makers want to capitalise on the public's interest.
IMHO, the chances of the KA3 film sequel happening is close to none due to the poor box office earnings of KA2.