3rd GPU on Gigabyte 970A UD3 possible??

Hi all,

sorry for the nooby question. Iam new and still learning. Unfortunately i cant detect the 3rd GPU on my Mobo which i connect via USB Riser cable. It only show me 2 of 3 GPU in the device manager,but why? What do im wrong? All cables are connected safe. may i forgot something? Im really appreciate your help.

The board has
PCI EX1_1 *new* third GPU >> Asus r9 280 3 GB DCU2 (Fan on- show in Device manager)
PCIx16 first GPU >> Asus r9 280 3 GB DCU2 (Fan on- show in Device manager)
PCI EX1_2
PCI EX1_3
PCIx4 second GPU >> Asus r9 280 3 GB DCU2 (Fan off> not detect in device manager)
PCI
PCI


When i disconnect the third GPU which i connect in PCIEX1_1, the two GPUs(first and second) which are connected direct to the Mainboard will work normally.

PCI EX1_1 third GPU >> Asus r9 280 3 GB DCU2 (DICONNECT)
PCIx16 first GPU >> Asus r9 280 3 GB DCU2 (Fan on- show in Device manager)
PCI EX1_2
PCI EX1_3
PCIx4 second GPU >> Asus r9 280 3 GB DCU2 (Fan on> show in device manager)
PCI
PCI


Update: All three GPUs are connected but the System show only 2. The fan of the two GPU( which are nearest to the CPU) works. What should i check next. buy another am3+ Mainboard which?

SYSTEM : AM3+ GIGABYTE 970A UD3 v.1.1 (GB RAM)
3 ASUS R9 280 DCU2
Am FX4100

I stuck to long on this issue. hopefully someone can help me out. Thank you in advance..