I’m sorry, I’ve just seen this. I think you mean me with the CRV and 3 seats?
From memory (and it was a looong time ago, as my youngest just turned 11!), I had:
dd1, who was 8 at the time, on a high backed booster - Britax Adventure? That name rings a bell. It was not a particularly fancy seat, but was (at the time) the slimmest hbb available. Don’t think it was isofix.
dd2 was 6, and was in the middle seat in a rear facing toddler seat. It was a Britax again, think it may have been the Two Way Elite? Secured using the middle seatbelt, and with restraining strap supports around the two front seats.
I can’t remember what ds was in as a baby, but it was an isofix base with the seat detachable - Pebble rings a bell, but really can’t remember, sorry.
I made an appointment at the InCar safety centre in Milton Keynes - we spent all afternoon there trying out various combinations of seats, and the above was what worked for us. The website is here. They were so helpful, and really knowledgable, so if possible I’d recommend a visit (I think they have other sites, but Milton Keynes was closest to me - still a couple of hours away, but well worth the visit as meant I didn’t have to change my car!)