Probably,
when I ordered my 330d I didn't choose a third buckle/head rest, but I got one nevertheless. I was sursprised by that because I didn't choose it but got it AND had to pay for it. My dealer told me than that in the Netherlands (where I'm from and where I bought the car) new cars need to have that by law but that it's indeed not standard for the 3-series.
I can imagine a third buckle isn't standard for an ' American' 5 series either. Try to look on bmwusa.com or (if you have one) in the options list of a 5 series brochure.
