No you cannot. This has nothing to do with the MST Hub thouch. A DisplayPort 1.2 output only has enough bandwidth to output 2 x 2560*1440p @ 60Hz. When you put the 2 screens in 90Hz, that will take more bandwidth of course (9,95Gbps per screen if i am not mistaking) so you can still do that. But 3 will for sure not work since you only have 21.6Gbps available.