Headscratcher...
06 Shooter--I have a similar set of RCBS dies, only mine are vintage 1971. (Yes, the "76" on yours is the year of manufacture.) Got mine @ a gun show, too. I've been loading since before 1971, but started with .30 Carbine only just recently.
My #3 die, which calls itself the "seat" die, also removes any bell-mouth that #2 put into the case. It also imparts a taper crimp around the bullet.
I haven't had any issues with loose bullets in the .30 Carbine ammo I've loaded with this die set, which would seem is the same setup as yours.
Either you are doing something 'way wrong yourself, or your die is defective. I can't figure what you might be doing wrong from your description, and RCBS has been making dies that just plain WORK, since they started, so I hardly think it's the die's fault either.
A call to RCBS customer service might very well be in order.
Hope you find out SOMEthing, and please keep us posted! Good luck on this one!
ETA--Had another thought: What is the diameter of the bullets you are loading? Are they truly .308"?? Not just a nominal .308 because it says so on the box?? The one time I had trouble with loose bullets (but not with .30 Carbine!) I discovered that some manufacturers make their bullets a smidge smaller than other manufacturers. Had to modify my case neck expander so the necks were a thousandth or so less in inside diameter--Problem solved. With the .30 Carbine I wouldn't modify the die, I'd just find a maker of a slightly fatter bullet, if I had that problem (The carbine doesn't seem to care much, what kind of bullets nor what maker, you are loading--Mine at least shoots all I've tried, just the same.)