The SL3 is an awesome shelter but IMHO for 2, not 3. I like some extra room and originally went with a RS2 for me and my dog and for possible use with 3 people occasionally. It was great and I highly recommend it, but the SL3 works a little better for me. It's heavier as a complete shelter but has the flexibility to just take the fly, which a lot of people do. Where I live, bugs are pretty manageable with good camp site selection. So I never take the nest or the really heavy pole. I use a 1.5oz pole extender with a trekking pole. And I use a 1 person sized piece of Tyvek for a ground cloth. With the fly, the extender, ground cloth, stuff sack, and stakes, the whole thing is just over 31 oz. That's pretty light for a sizeable shelter if you don't need the bug protection. I have a thick coated dog that stays as wet as he possibly can. No floor means no no messy, muddy, dog hair covered floor. That's a big plus for me. It's really just a shaped tarp without the nest but a very nice shape. I have a bad back and bad knees (hopefully only one as I'm recovering from a partial knee replacement) and a light chair is not a luxury item for me. With the SL3, I can easily sit in my Slinglight inside (see my avatar pic). Very nice to sit and read during a rainy night rather than laying down for 3 or 4 hours before sleeping. I always hated that.
I'll admit that what I value in a shelter is probably a lot different than most people, but since you asked, that's why I like the SL3 so much. But my youngest is starting to want to come along and if she does, I'll regret having sold the RS2. The CB3 looks like it might be just the ticket for those trips. If I can keep the dog dry.