In regards to the HDMI testing+ each DTGM30 module has two outputs+ so why do I need 3 DTG modules to generate 4 TMDS signals?

In regards to the HDMI testing+ each DTGM30 module has two outputs+ so why do I need 3 DTG modules to generate 4 TMDS signals?

Slot A of the DTG has few additional features. They are :

Please note that whenever you use the above special features, you cannot use CH2 of Slot A.

In Cable eye diagram testing we need to introduce jitter in Slot A -CH1, because of this CH2 of slot A is not usable, so you need another module to generate the forth channel.