The data sheet says this:
The Cisco Nexus 3500 platform is compatible with existing Gigabit Ethernet infrastructures. Both the uplink and downlink 10 Gigabit Ethernet interfaces can also operate in 100 Megabit Ethernet and 1 Gigabit Ethernet modes. Table 5 lists the Gigabit Ethernet SFP transceivers that are supported. 100 Megabit Ethernet connectivity can be achieved by using copper-based SFP transceivers (GLC-T).
