Yes each film declined domestically, but that's somewhat misleading.
First off SM1 was one of those once in a lifetime releases. We've only seen that kind of audience response from TDK, and Avenges. We've never seen that kind of response in the genre from the first film in a series (I'm counting Avengers as a sequel, because it sort of is). It broke the domestic weekend record, which it held for 4 years until POTC2 came out. Secondly there was a huge draw to the film with the aftermath of 9/11, and the positive message of the film. Thirdly let's be honest here Attack of the Clones was a MAJOR disappointment, and whatever competition it may have presented pretty much fell by the wayside.
SM2 was released on a Tuesday to capitalize on early viewers before the 4th of July weekend. It ws 374, compared to 403 for SM1, so it wasn't a huge falloff, and certainly wasn't due to the quality of the film as SM2 is generally considered the best Spider-man film to date.
SM3 regained the opening weekend record over POTC2. The response was hug despite negative reviews. However once people saw the film, that's when the fallout happened. SM3 had terrible legs and only amassed $336, which was barely more than Iron Man made the following year on a much smaller open.
TASM was definitely down, but I think this was expected. Even Batman Begins didn't have great box office, but it did have strong legs at the BO. It also was effected by the Aurora shooting as it had a near 70% drop the weekend of the shootings.
This takes us back to TASM2. So with all of those things, the expectation would be for it to finish above it's predecessor, but it's going to fall far short of it. Had it not been for the Aurora shootings TASM could have been over 300M, so that makes $220M for TASM2 even more depressing.