Why it bombed in the US
I think that Rambo 3 wasn't as successful in the US,because it hasn't dealt with an American problem,unlike Rambo 2.The Americans don't care what goes on outside their own country.You could clearly see that from reading some of the reviews for R4.Even the critics couldn't care less,let alone the average movie goers.
What the the Americans don't seem to realize is,that Rambo is an international icon,known and loved all over the world.Rambo already became popular with the release of First Blood way back in 1982,which was just called Rambo in the rest of the world..And he grew way more popular with the sequel.
Rambo 2 was the most successful because of the patriotism,general political situation which was going on in America.The American audience was craving for a movie like Rambo 2.It was USA vs Russia.Just like Rocky 4,which also became a hit because of that reason.Even the producer of Rambo 2 said that if the movie came out 2 years earlier, it wouldn't be nearly as successful.