I would probably rec the CA as a semi-narrow FFing seat. It's not Radian-narrow, but it's a lot easier to adjust.
TBH, I would have a really hard time making recs based on that situation. It's probably due to my own experiences -- what I need for share-able seats and 3-acrosses doesn't help. I would be more likely to explain to a dcp that they're going to need to look at the kids they have and vehicle and be aware of some options. What works for "now" may not work six months or a year from now, as kids grow or as different kids fill their spots.
Taking 3-across OUT of it: EFT65, TF for RF seats, TF/CA for FF seats, Turbo for HBB and Literider/Cosco Highrise. I might consider recommending a CCO if the dcp often has infants and is not married to a bucket, as a narrow alternative to the Radian. If 3-a is a frequent necessity, I would rec a Radian and suggest the dcp put a full-time kiddo in it. (Most dcp have some f/t and some p/t kids, so they could do something like a RN/EFT65 plus a third seat, and use the EFT for the shared-spot kids.)
I might -- might -- mention the SmartSeat, if I had a dcp who had kids of vastly different ages sharing a spot. If it had come out even six months sooner, I would have owned one just for that reason.
Sorry, I got a little long and rambly... hope it helps.