I mean technically, we did get a 3rd. We gave up Rasul and a R5 to get it.

They'll play Rasul at CB, not S. They lost TreDavious White to injury, and Elam has been a R1 bust. They need competent players in the secondary. BUF has been hit by injuries. Milano is out for the year and they just lost someone on the DL, IIRC.
Long term he might be a good choice at S. He's smart and has size to play the position, but it would take some time to learn how to play it. Not something you do with a veteran mid-season.

I find it hard to believe he's the #10 CB according to PFF. Their grades are hardly perfect, but generally they are spot on. He hasn't been bad, but there have been a few times where he's been burned this year, and I think that will only increase as he ages.
His contract was pretty good considering his performance, so BUF is getting a good player for not much. They only have him for 1.5 seasons though, and Day 2 picks have value for the 4 year cost control on the contract if nothing else.

His comments to the media and locker room presence were NOT a reason to trade him, everyone has said as much. Rasul didn't say anything crazy that would get him traded, and he didn't pull a Ty Montgomery or anything. This was just business.

I'm a little surprised people are so upset about this. Yeah, he's still a good player on a good contract, but GB has players in Valentine and Stokes they want to get snaps and value out of.
Rasul being on the roster doesn't make this team a SB contender. He'll be gone when the next playoff contender window opens for this team, might as well get some value for him now.

I think the Bills got the better side of the deal, assuming the R3 is cursed. If they end up getting the next Aaron Jones or a very good 8 year starter at RG out of the draft with the pick then GB is the clear winner.
(What if they used the cap space from trading Rasul to sign Saquon lol?)

Personally, I'd prefer Rasul be on the roster, especially with the Rams coming to town. He's gone, they got some draft capital and cap space for him, let's move on.