why can i only get 2 screen to work .i have 2 cards 3 screens . i have the right cables

i have 3 screen all runing via a matrox adaptor and have been for many year. i have bought 3 new screens with hdmi . i aslo have 2-cards R7900 bit old but ok . what i cant seem to work out is runing in 3 screen only 2 via eyefinity  i have tested with all type off adaptors