I assume you mean 3-pin connectors to power case fans, not the 3-pin PSU fan sensor connector for the PSU itself. In that case, no; I've never seen that. I don't think it's cost-effective to include additional wires just to run low-current devices like case fans. But beyond that, the intention is that any variable speed case fans you have will be plugged in to the 3-pin headers on the motherboard dedicated to that purpose. This enables software monitoring and control of the case fans in response to system temperature sensors, or whatever. If you have more than two variable speed sensor fans, you're SOL with most motherboards, though.
Mookhow wrote:
edit: With drives switching to SATA power connectors, are Molex connectors only being used by case fans at this point?
Pretty much, yeah. That and other random powered case accessories mainly.