I just checked, the fan headers are labeled A1, A2, A3, B1, B2, and B3. It seems like only the fans that are plugged in in an ‘A’ slot are shown in the BIOS and therefore controllable (3 pin and 4 pin).
I currently have my two front intake fans connected with a splitter and plugged in an ‘A’ port. My bottom intake fan is plugged in an ‘B’ port and my rear exhaust fan is plugged into an ‘A’ port as well. Two of my 3 top exhaust fans are connected with a splitter and plugged in an ‘A’ port and the third Fan is plugged into an ‘B’ port. It also seems like the fans in Port A1 and B1 same with same with fans in A2 and B2 are running at the same speed, could this be possible?
PS: Is it possible to connect 3 fans with 2 fan splitter into one 4 pin port?