is it possible for a Kensai with 2 weapons attached to equip a third destroying one of the ones already attached?
Attaching a new weapon to a Kensai with 2 weapons.
A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
is it possible for a Kensai with 2 weapons attached to equip a third destroying one of the ones already attached?
No.
You may not attach a third weapon to a Kensai with two weapons attached - you may not attach a second weapon to a non-Kensai with one weapon attached, either.