Why are my two smaller goldfish attacking the larger one?

I have had these three in a small 'spongebob' plastic tank for 12 months, with no problems. Yesterday I bought a larger (5 Gallon) tank for them, set all the ornaments up in the same locations as the old tank and moved them in. As a treat I gave them their first taste of live food, with brine shrimp, then those other little jumpy things (I forget the name). They loved the food and then seemed to be relaxing in the new tank, but as I went to bed last night I noticed that the two smaller (2 incheslong) goldfish were chasing the single larger one (3 inches) around the tank and it was trying to get away - they are still doing it today - they seem to be going for the gill area, and are pushing it around, sometimes one on each side, sometimes, trailing slighty behind. What's going on? They were always friendly before.