In reply to Getoiu:
You will probably get different responses here too!
I suggest a minimum length of 30m for 3 people but only if there are other folk around to assist with any serious crevasse mishaps. I use a 50m 8mm half rope for such things to save weight. It is dry treated, although I'm not sure if that really helps.
Obviously a full weight rope would be a better buy if you wanted to use it for more serious things later.