I know that my r9 295x2 supports it via the mDP with a proper adapter.
However, I recently purchased an msi r9 290x gaming and I wish to do a tri-fire setup with my r9 295x2.
Will the r9 290x support hdmi 2.0 via the hdmi 1.4 input?
No, it would be impossible for a HDMI 1.4 output to use HDMI 2.0. Otherwise it would be a HDMI 2.0 port not a HDMI 1.4 one.