PCIe Slot 3 is connected to CPU2. You need to have second CPU installed to get PCIe slot 3 enabled. 
I have just bought a Broadcom 57412 Dual Porte 10Gb, SFP+, PCIe card from Dell for a PowerEdge T440. Unfortunately the card is a x8, and I have no x8 free slots on my T440:- slot 1 x4 is free- slot 2 x4 is free- slot 3 x16 is free- slot 4 x8 is in use by PERC Raid controller (x8)- slot 4 x8 is in use by BOSS controller (x8)I tried to put the card on slot 3 x16, but it does not work: iDrac can detect it as "BRCM 10GbE 2P 57412S Adptr", I can see the transceiver manufacturer (TPlink) on iDrac, but then no firmware info is available and no PCI ID is assigned. ESXi can't see the NIC with lspci.After reading the documentation, I discovered that slot 3 is a kind of a special slot, in which I can add a GPU only when a second CPU is installed.[URL] table 17 and table 18 on installation and service mannual.I have a single CPU.What can I do to enable this new card ?Is there an option different than throwing away the new NIC or the BOSS controller ?Thank you
PowerEdge T440, new network card PCIe x8 in x16 slot