You can try to setup one as the client and have that client feed to the other mining rigs so that it only downloads the block chain once. Thats how mine is setup and i barely notice that issue.
I thought that mining doesn't need to download the blockchain? Unless you're mining solo, that is. For pooled mining, you're essentially sending a couple of kilobytes per second only, hence going over 5GB would be nearly impossible unless you have a mining farm.