People blaming Resistance 3 "failure" on Resistance 2 are not correct imo and I feel I have to defend Resistance 2 because I really don't understand the hyperbole and vitriol thrown around when talking about it.
Only hardcore fans (or PS3 only shooter fans in general) know the design differences between Resistance 1,2 and 3. Hardcore fans read up on this stuff, they go on gaf, they go on forums, they read interviews pay attention to E3 etc. and so on. They know that Resistance 3 fixed a lot of the 'problems' of Resistance 2 (no weapon wheel, different atmosphere/story direction than the first game, 2 weapon limit, etc. and so on whatever was considered bad about R2). So how could the failure of Resistance 2 damage R3 sales if all the hardcore fans knew R3 wouldn't be like R2? They would already know that Insomniac paid close attention to fan requests and changed many things in favor of them.
I believe the reason for Resistance 3 "failure" (or just not selling gangbusters) is one or more of the following:
a. FPS/shooter fatigue, people are tired of it. Resistance came around at the worst time possible imo when there was a huge influx of shooters and the series got lost in the crowd. Now the people that AREN'T tired of shooters, are fans of point B.
b. COD and/or Battlefield. Casual shooter fans are interested in these franchises (I'm not saying Battlefield is casual before I get shitted on, but it's just a shooter more casual fans know about is all) These are the staple franchises, nothing else is going to make a dent into them without huge advertising and more importantly doing something different/unique which gets into point c.
C. Resistance in my opinion doesn't do enough to set it apart from other shooters. It's no Bioshock series. Bioshock is my favorite game/series this gen because it pays so much attention to story, atmosphere, characters etc. and the gameplay is a wonderfully fun cherry on top to all of that. It's so different and great that it has it's very own demographic and doesn't have to appeal to COD players. I feel like Resistance should've been this kind of series (although there was no way of knowing that military shooters would be so dominant so it's not necessarily Insomniac's fault).
D. I bought Resistance 1 and 2 day one. I enjoyed R2 much more than R1. I played Half Life 2 when it came out, Doom 3, Halo etc. so Resistance 1 to me felt like everything I had already seen and the most generic, dull kind of game out there. I didn't like R1 at all but was forcing myself to play it because PS3 had nothing else at the time. I jumped for joy when Rainbow Six Vegas was finally released. Now some good things were left out of Resistance 2 that I felt shouldn't have (weapon wheel, vehicles, health packs) but R2 I felt was really good and does not deserve the hatred it gets. The gameplay changed to a more COD-esque kind of thing whichw as definitely lame but not a deal breaker. Aesthetically R2 was so much more pleasing to me than R1's dull gray everywhere. And gray is okay but only if the art style is beautiful enough to supplement it and R1's just wasn't in my opinion. R2's use of colors and the time period etc. I thought was great and really enjoyed. R2 wasn't the end-all to shooters but I liked it much more than the first (even with the COD gameplay which I wasn't fond of but it's something everyone was used to).
Now I was interested in getting Resistance 3 after it was first announced I was really hyped about it. The initial teaser was amazing and I felt like this was going to be a real uprising, revolutionary etc. kind of storyline where you'd team with rebels and take down the aliens and so on. It would be a serious, goosebump-inducing kind of thing but these days I can't justify spending 60$ just for single player unless the single player is it's main thing (like Bioshock, Assassins Creed etc.) with lots of effort and time put only into the single player.
So the decision to get R3 rested on whether or not the multi player would be interesting/unique enough to play. 16 player COD-esque stuff was announced and it immediately killed my interest. I like COD but I have no interest in playing copycats especially ones that aren't 50-60fps. If the game had 40 player multi player, something along the lines of Unreal Tournament etc. stuff like that I would've gotten the game. I still really want to play R3's single player but not until it reaches 30$. 30 for single player 30 for multiplayer that's my separation and I wish Sony would allow some kind of thing where they sell the single player on PSN for 30 bucks because I (as well as many others who don't care as much about multi player) would bite on that deal for sure.
ninja edit - and sorry for the long post I guess I was just bored of reading constant 1 liner responses of people saying R3 deserved to tank or R2 was the worst game ever etc.