Guinea pigs all the sudden fighting!!! URGENT?

About a month ago, me and my two brothers each bought a male guinea pig from PetSmart. The three had been fine in the cage at the store. The largest one, mine, was brown. The medium one, my older brothers, was light brown with white specks. The smallest one, my little brother's, was white with brown specs. The guinea pigs have separate cages but visit almost every day. A few days ago, while all three of them were visiting in one of the cages, mine and the smallest pig turned on the medium one. We separated mine and the smaller one from the medium one. Tonight, I just put my and the medium pig together. In the first minute, they were biting at eachothers throats!! My guinea pig is getting fat, and all the pigs eat the same food. Any explanations?????