Hi!
You cannot run mixed mode. The spanning tree mode affects the mode of the entire chassis.
Further you cannot have pure RSTP on 6500. 6500 supports,
Ferrari(config)#spanning-tree mode ?
mst Multiple spanning tree mode
pvst Per-Vlan spanning tree mode
rapid-pvst Per-Vlan rapid spanning tree mode
So you can either run rapid-pvst, or if you dont want volans 3 & 100 to have different spanning tree instances, you can run mst and combine them in a single instance.
Regards,
Niranjan
(pls rate helpful posts)