I actually enjoyed the third film. I think that critics sometimes get wrapped up in expecting series to evolve in a critical way, and forget that series like MIB were always going to be stupid in the first place. The first movie was better, yes, and so was the second. If you busy yourself with how "disappointed" you are with this movie, after placing such expectations on a sequel, you've obviously been expecting more than is realistic.
It's a VERY RARE third movie that isn't a planned part of a trilogy that doesn't turn out a bit "meh". I think of this as an average movie, not a bad one. Emotional reactions of disappointment are certainly what hardcore fans of the franchise may have, but everyone else knows it's just a stupid film to take your family to see.
I'm not saying recommend the film, but don't start calling something terrible simply because it did not meet expectations. That's bull. It's not terrible, it's average. Very average, lots of wasted potential as well. However, I got laughs out of it. I'm sure you could do much worse with some other movies that are on. A few of those really are terrible.