There are, I believe, 5 maps in total for the Iron Banner, and all games are Control. Obviously, the aim is to keep at least two of the three flags, but I'm pretty certain that kills do also count towards the scoreline. There have been a few games where I felt like we were cruising it, holding two flags almost all the time, but the scores have actually been quite close, as the other team must have been getting more kills.
In terms of maps, I reckon 3 of them are massively unbalanced in terms of flag/spawn locations. For these three, your only aim should be to capture and defend the following flags:
Firebase Delphi - A+B
Shores of Time - B+C
Blind Watch - B+C
The winning team is almost always the one that keeps these two flags for the longest period. The two flags listed are the closest together and so far easier to defend.
The only occasions you should ever look to capture the other flag (the one not listed above) are: (a) you are winning easily, and I mean easily, and having no problems holding the other two or (b) you spawn there, in which case, you should capture (only 1/2 people should stay behind) while the rest of the team heads to B. If that fails, and the opposing team captures B, you will all respawn at the original point, in which case, everyone should rush the furthest flag (i.e. not B, as B is normally more heavily defended) and keep doing that until you capture it. If neither (a) nor (b) are true, you should not capture the 'other' flag. Even if the other team captures all three and you spawn by it, do not try and recapture. If you do, it will take you a while and it will give you very little benefit. Your team will repeatedly spawn there and you'll all have to run a long way to attack one of the good flags. You're better off dying with no flags held, in which case there is a chance you'll spawn by one of the good flags.
If the game is close and you have control of both good flags, do not try and capture the third! Just defend, defend, defend. Games can turn when someone thinks they're being clever capturing the third and, in the meantime one or both of the good flags are lost. I can't emphasize enough, capturing the 'other' flag on these three maps is more likely to lose you the game than it is to win it.
The other two maps are more balanced. For Twilight Gap, there is still an advantage with A+B, as there is a quick route between the two that makes defending easier. But I have also been in and against teams that have succeeded in winning with B+C, or even A+C.
Finally, Rusted Lands is by far the most balanced map. A is the closest flag to B, which means you can defend the two well enough as long as you have a couple of people that can stay alive and get kills. However, the spawn point for A is down some stairs at the back of the map, so it's quicker to get to B from spawn point C. On this map, I would say defending the flags you have is still preferable to going charging off and trying to capture the third flag, but you can still win by doing this, particularly as capturing flags gives you super energy.
Talking of which, try and save your super abilities for occasions where you are capturing or defending a flag. Even if it means dying in a 1v1, you won't regret it when 3/4 opponents think they're capturing an easy flag and you run round the corner and pound them all to dust.