Their reasoning for not having 2 keyholder is that 2 people are more likley to colude to change the results (in this case announce false results) than 3. Of course 3 people could still colude to do so, so it's a matter of reducing not eliminating the risk. My understanding is that in 2 out of 3, the third can also decrypt/view the results, so (assuming number 3 doesn't lose their key) then 2 can't colude to cheat (unless they also colude to somehow "deprive" number 3 of their key (e.g. with a heavy wrench)). If number 3 does lose their key, then the risk of colusion is higher than "requires all 3", but conversly the risk of "accidental or deliberate failed election" is lower. It's (always) about a balance of risks.