I can only answer for how two bunnies respond when the third member in the trio dies, so I cannot compare it to what happens when one of the bunnies of a pair dies. However, I should clarify that every bunny is unique, so how one bunny reacts is not the same way another bunny would.
Anyways, when Jack was put to sleep in April, both Karl and Molly reacted strongly to it. Molly was crazy thumping all evening and night and running around frantically without pauses. It was impossible to get through to her. I gave her something to calm her down if I remember correctly. Karl changed behaviour as well. He was afraid of us and it was difficult to get near him for about a week, but a few times during these days, he came over to be petted and would ask for it for much longer than he usually does. So he clearly wanted cuddles, but his nervousness made it impossible for him most of the time.
Also, my trio used to snuggle quite a bit, but still do lots of things on their own. But after Jack’s death, Molly and Karl were attached to the hip. One could not go anywhere without the other. This went on for about a month or so.
The reason I actually got a trio in the first place was because I read how strongly bunnies react to their mate’s death, and I must say that I was happy that Molly and Karl had each other considering how they reacted.