AM3 boards with AMD chipset do not have driver support for SLI. The 990 chipset boards ushered in SLI support with AMD again.
EDIT:
There were but maybe no longer available, a few AMD cpu boards with a wonky Nvidia chipset that did support SLI. Not many of those boards were made. MSI and Asus made a model or two. If you overclock, I would not have later AMD cpu and older Nvidia chipset mobo. They are most often cheap boards and in general, they overclock poorly.
END EDIT.