What are 'personal letters of...Adlerauge/Clavius/Michael The Stalwart' and do they have any use?

Question

every  personal letter of... have the same appearance and three parts. The names are assigned to mini-bosses that appear in the new quest, The order of The Stag... but how to obtain them? They are useful for something?

Answer 1

I and II dropped from minibosses on Ada. Never seen III part. Maybe its rare drop. Idk

Answer 2

So far no use found of these letters…

We can drop part I and II from bosses and one of them is obtainable in a chest but no one found the part III yet