In general, it is not really necessary to swim overlong long distance sets, as there is no real training benefit other than mentally knowing that you can swim the distance. Even for competitive swimmers doing the 1500, workouts will rarely have that distance in the plan.
There is more benefit to swimming a set such as 6x500 on :10 rest than there is in swimming 2x1500, or 1x3000, and if you can complete a set like that, then there shouldn't be any problem in being able to swim the distance straight.
Example for clarification: If you have a 3000m race, and you are thinking that you can swim that in 1 hour (60 minutes), then you should be able to do 6x500, at the pace of 10 minutes per 500 (2:00/100) with a :10 rest interval. If you can't do 6x500 on 10:00, :10 rest interval, then your race pace is too ambitious and you need to aim for a slower pace. If you simply can't even swim 6x500m, then you have larger problems than worrying about pacing.
If I were training for this, I would make sure that my workouts in total were more than 3000m, and that I had longer (400-800m) components to each set, but I wouldn't be overly concerned with set components longer than that.