I know it's something easy and I am sure I have done it before but how would one create traffic on a a different NIC (i.e. Eth3). Is there a ping option for it?

By "create traffic" do you mean move traffic to a different NIC?

Please elaborate more on what you are trying to do.

man ping.

Try ping -I [interface]

Note that not all versions of “ping” support this option. In particular, I don’t think GNU’s own version (à la inetutils) supports it. Some distros use the version from inetutils and some use the version from iputils (which does happen to support the option). The *BSDs support the option to varying degrees. I think Sun’s ping supports the option as a lowercase “-i” (opposite from the others).

You might also “create traffic on a second NIC” by temporarily changing the routing tables. Of course this usually requires superuser privileges (but for that matter, so does ping).