Not sure really. It all sounds good and I like the idea of picking a gold, silver and bronze but if the maps are as cut and dry as you say then surely all this would do is mean the gap between first and last would be even greater? Or are you saying you would prefer someone to get 3 votes no matter how many entries there are to try and separate the group bringing up the rear? If so, why not just vote on every single map, putting them in order of preference? If that was possible then it would be compltely fair.
I have a feeling though that it might be a reduced amount of votes to facilitate casual voting. Someone with a spare few minutes can quickly vote on one or two maps but it might take some thinking to place all in order of preference. I know that isn't what you suggested but I would view that as the best system.