I'm assuming if you're all together then you'd take husband's car? In which case you should be fine getting all 3 kids in yours - put a RF seat in the front with the airbag off and HBB + other RF seat in the back. The Avionaut Sky Q would be a good one to look at because it's very compact, not the longest lasting but should be fine until safe HBB age (5½-6yo) if LO is small. For your husband's car, ideally the best option is to visit an independent retailer who specialise in car seats. If you look at the retailers list on the Axkid website that's normally a good place to start. You'll probably be best with belt fitted RF seats on the two outside seats and a slim HBB in the middle. If you're really struggling you could potentially get something like a bubble bum for 6yo for the middle. It wouldn't be as safe as a HBB but better than nothing
If you go on a family day out take his car. If you need to take three three kids in your car put one child in the front seat
360 seats are really big you're right. You could put the baby in the infant carrier in the front and sit an adult between the seats in the back? Or does the 6 year old not use a seat anymore? The best thing to do would be go to a specialist car seat retailer so you can try different combinations in your car.