Mantha has been good this season, and since he is UFA it should not matter at all what he did and didnt do last season or two. He is not a cap dump anymore, he is a ES-25 goal winger. With 50% retention he is cheap to fit under the cap too. And as a rental its pointless to say ”contract year” or above average shooting%, we are not valuing him based if he can sustain his current level for next season. All that matter is what he is today.
3rd alone isnt enough and Mikheyev has no value for Caps and they arent there to make you favours.
We already have our own bottom6 wingers, and he is not what we need for our top6. Thats just blocking capspace for real help and blocking roster spot for someone else. If you need him moved you move him elsewhere in a separate deal, or you pay extra for us to take him.
So its an easy no for somethting like Mikheyev+3rd. Straight up 2nd is the least id trade him for, and adding an cap dump (even if the player isnt really bad) + retaining only increases the value.