hehe....simple logic problem...
answer: 3
because in that case everyone observed 3 sick dogs from others
except for those 3 people who own the 3 dogs
they observed only 2 sick dogs
and every one of them will think: if my dog is not sick, then ar the second day,
the two pwople will kill their dogs
and that didn't happen
so the only possiblity is that his dog is sick
so at the third day, 3 people killed their sick dogs