can sum one help me setup a triple monitor with 2 moitors mirrord and have the thertd on as a seprat monitr

i have 2 monitors and a imac im trying to set up one of my monitors as a separat monitor and have the uver one mirroring my imac but i can only get it to mirror my mac on all my screens and i dont wont this and if it help but i dont no if it make a difference one of my monitors on useing hdmi and the uver is useing

vga if eny one can help me i will be very gratfull