yekke2 –
When you go into the shop, do you ask for a kilo of matzos or a box of matzos?
It doesn’t matter; you are buying by weight. The idea of davar shb’minyan is that the individual thing has a chashivus that does not allow it to become batel. Since people don’t sell them individually, but rather by weight, they don’t have that chashivus.
That isn’t bitul b’rov, that is a different type of Rov where you are ???? ??? ????, where the ??? is ???? the ???. You probably have a problem of ???? on this.
Bitul would never take effect in a case where kavua was shayach. All dinim of rov are essentially the same. None are a birur, all are a din. Kavua would not take effect here because the issur and heter are completely indistinguishable one from another, just like three pieces of meat are.
Agav, the question of whether bitul changes the status of the thing being nisbatel is a machlokes Rashba and Rosh. The Rosh says this, the Rashba says no such thing. The Rosh isn’t saying that the rov is mevarer the safek, he is saying that the din of rov says that the mi’ut is nishapech in din. The Rashba doesn’t hold of this, but he hold you can eat each one because on each you can say “I am eating from the rov.” No din of birur.
I don’t mean to get all patronizing, but the essential reading material on rov and sfeikos is not first and foremost the Shmatsa and R’ Shimon. It’s the Rashba in Toras HaBayis in Sha’ar Hata’aruvos (and of course the relevant Gemaras in Zevachim, Chulin, Beitza et al with the basic rishonim). My point is that I do not think you would have made this point if you would have seen the Rashba.