Nice. I forgot about Federation Force (mostly I just remember the controversy with it not being what fans expected or something), but I have never heard of either of the others. I will have to look into them.
The 3DS has such a wide range of genres (although it is heavily biased toward RPGs and Platformers) that I’m not surprised there are more than one FPS for it. Wasn’t there one called Ironfall? Or was that one was 3rd person?
Unless you consider Ace Combat an FPS, the 3DS doesn’t really have any of them that I know of. You’d be better off playing the old DS ones. If you count Homebrew, your options expand a lot. Doom and Quake play wonderfully on the New3DS with the C nub.
As far as my favorite games on it: wow, there are so many. Shin Megami Tensei IV (and Apocalypse), Fire Emblem Birthright, Animal Crossing New Leaf, Pokémon, Monster Hunter 4U. The list goes on.
Making friends on sites like this can sometimes yield invites. There are also usually communities specifically dedicated to giving out invites. And if all else fails, some trackers have open invitations (either time-limited or with some kind of interview).
A common method is to get your feet wet with an easier to get into private tracker and use the invitation threads in it to get into the ones you want.
The organization and typical submission requirements are what really put them over public trackers for me.
Public tracker: It’s this big and this many files. Figure it out.*
Private tracker: All the metadata
* Experience may vary. Post is overly dramatic for comedic effect
Looking at some screenshots, that may have been Dementium Remastered as mentioned above by SturgiesYrFase. I’m not sure if it got a demo, and I hadn’t heard of it until about an hour ago, but it’s a guess.