I shoot a little 3 gun here in CO. I would say, take what you have and see what others have/what works for them before you buy based on interwebz advice. For instance, I shoot a Glock 26, an AR and my Rem 870. Are those the best guns for this competition (as in scoring the best times), no. I shoot my Glock 26 because that is what I carry and I want the competition to act as a form of training for me, I shoot the 870 because that is the shotgun I have and I wanted the most durable gun I could get (and then train to beyond proficiency with it). I know that not everyone at my shoots has that same mentality...some are there to win (and there is nothing wrong with that).
With that being said (and take this with a large helping of salt), IF my main goal was to win (score the best times), I would likely shoot a high cap 9mm (Glock 19, 17 or similar; faster follow up shots, more capacity, cheaper ect ect). For rifle, ARs with some form of red-dot seem to rule for the same reason.
Shotgun: Not sure if all 3 guns are this way, but in my limited experience, the shotgun is where matches are won and lost. Those who score high all have simi autos...these range from supper high dollar Benellis to now discontinued Winchesters (and all with extended mag tubes). Time is lost to the greatest degree during reloads, and a shotgun tube is slower than any detachable magazine gun (we can't use detachable magazine shotguns at our shoots, check your rules as they may be different) therefore you want something with the greatest capacity. For “out of the box” pumps, the Mossberg 590 A1 is very prevalent (8+1 vs my 870’s 6+1...those two rounds have made a huge difference before). Also, along these lines, you may want to pick up a belt mounted shotgun shell carrier (like this-->
http://www.midwayusa.com/viewProduct/?productNumber=422576 )
Either way, in my experience, most of the guys (and gals) at 3 gun shoots are great people and have no problem letting other people try out their guns/ gear. Whatever you do, I would highly recommend you attend; they are a lot of fun with good people and a great way to induce some form of stress/anxiety into your firearms training.
edit to add: Not sure which FN shotgun you are mentioning, but I would avoid the one with the AR style sights. They look cool and all, but "nose to the charging handle" style of sighting would not be my preference on a 12 ga <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />