The Gemara goes thru a dicuusin of what is the mishna come to exclude when it says "3 ways" and being unneccesary we say it is coming to exclude somthing then the gemara goes into that.
But what about by a yevama the mishna does not say she can be bethothed in one way and divorced in two ways and yet we dont have to get into a discussion about things that you might would work but do not? (for ex moeny and shtar)?
Daniel, ny
In fact the Gemara does ask (below 14a) that a Yevamah should be acquired to the brother-in-law through money and a Shtar just like every other married woman. The Gemara answers by citing a verse that it is only "Bi'ah" that acquires the Yevamah and not money or Shtar.
The Gemara there continues and suggests that the Yevamah could leave her husband with a Get and that we could learn this from a "Kal v'Chomer" from a regular married woman. The Gemara answers by citing a verse that a Get does not work for a Yevamah because only the shoe of Chalitzah can exempt her from the brother-in-law but a Get cannot.
Kol Tuv
Dovid Bloom