Why a woman is pasul to be a dayan

Tosfos in Bava Kamma 15 raises the question of why a woman is pasul to be a dayan, as there is no explicit miyut anyway that disqualifies her. Tosfos ends up using a bit of roundabout logic: since a woman is pasul l'eidus, and we have a principle that anyone who is pasul for eidus is also pasul for din, m'meila a woman must also be pasul l'din. The Yerushalmi (Brachos 7 halacha 1) debates whether there is the possibility of zimun for 2 people, or can zimun only be zone by 3. The sugya draws a parallel between the halacha of zimun and the mechlokes R' Yochanan and Reish Lakish whether 2 people are called a Bais Din, or is a Bais Din no less than three. How are these two halachos connected? The Ohr Samaich in Hil Brachos writes that the underlying issue in both cases is one of minyan - at what point do indivuduals count together to form the "multiplicity" of minyan required for din or for zimun. The gemara in a number of places tells is that "trei nashi k'meah gavri" - women even in a large number do not count as a collective unit. Based on this, the Ohr Sameiach ingeniously suggests that since we know that 3 women cannot join together to form a "multiplicity" when a group or minyan must be formed, it follows that even if three women sat together they would count as individuals and not as a group of 3 required to constitute a Bais Din.