The 6-speed isn't going to be select-a-gear? If not, you realize that "3rd" (There are only 3 gears in a 4-speed auto anyway) gear is the highest gear, and O/D is usually a clutch band of sorts that locks up for that 4th "gear"? If you want 3rd, you simply turn the O/D off. Not sure how the GM works by still locking the torque converter up or not, but the 3 was probably replaced by the O/D switch. If it has both...the switch probably decides whether to lock or unlock T/C.
As for how a 6-speed automatic works? You got me. My Explorer's 5-speed automatic locks the O/D unit after 1st gear to give you a 2nd "gear" but it just uses 1st gear ratio with the O/D ratio added on to give you another speed range. Then you have true second gear (3rd speed), 3rd gear (4th speed), and then O/D locks up giving you the final 5th speed. Confusing? Yes it is, but that's how it works. Maybe the 6-speed locks O/D after the second gear too? Might have two sets of planteraies...that'd be a pain to re-build.
The 3rd gear he is referring to is probably the one that was available on the N/A S80s. There gears were P-R-N-D-3-2-1. However, the T6 version had a 4-speed manual (4 gears). The new S80s will have a 6-speed manual (all 6 gears) and not a transmission like that in the old S80 2.9