I'm having a thought while sitting here in the midst of some ambience mastering - it may be too late for this round, but maybe for the next ambience round:
Instead of limiting to only 3 recordings, maybe do it as "3 recording at most, consisting 15 minutes aggregate maximum"
Example being, I've got a great rooftop ambience which mastered and cleaned is exactly 10 minutes - and it's the kind of thing, like most ambiences of one location/POV, I don't like to "split" or throw away what's left after cleaning it - I find it to be a real PITA in the edit later when I have to track down all 5 parts to get all the sounds I want out of the total ambience. Soundstorm ambiences are a good example of this problem in my opinion, some good ones are actually 15 minute recordings they split out across many 3-4 min files and it's frustrating. I think that if a user wants to submit for example, a single 10 min ambience instead of two 5 min ones, that should be an option.
Basically any combination they want as long as they submit no more than 3 files, no files are less than 3 min, and whether they submit 1,2, or 3 files, the combined length of their submissions can't exceed 15 minutes.
I can only speak for myself here when I say that when there's a limit for an ambience recording, I'm always going to the longest end of that limit if I can, and people tend to be generous and will probably submit far more than a single recording - so I arrived at 15 min aggregate based upon 3 files x 5 min max each. Works out the same in the end.
I only just had this thought tonight, and sort of in hindsight (since we're near the end) - it may in fact give contributors greater creative latitude in a quality over quantity approach. Just a thought