Base power = (party member's base Attack) / 10 + 5

source

Weavile's base attack is 120, and each Kartana's base attack is 181. So the effective total base power would be around 120/10+5+5(181/10+5), which is 132. (note that I'm only rounding at the end, which might make this calculation inaccurate) I shall now demonstrate its power against the second best Pokemon in OU.

252+ Atk Weavile [132 BP] Pursuit vs. 252 HP / 252+ Def Quagsire: 178-211 (45.1 - 53.5%) -- 39.5% chance to 2HKO

source (beat up wasn't in the calculator, so I just changed pursuit's BP to 132)