DFS Replicate group with 3 server

Dear Forum,

i am crazy with our DFS servers, currently we have DFS with 3 Server(DFS01,DFS02,DFS03), i noted that sometime it active the primary server on DFS01 sometime on DFS02 and sometime on DFS03, 

i would like to ask that , how we can set the permanent to DFS02 is primary ?

Thanks you!