We just hired a Falcon XR6 (sedan) while on holidays and were amazed by how roomy it is (both inside and in the boot). I'd highly recommend one (the XR6 isn't necessary though, that was an unexpected bonus with our hire, lol) and don't think you'd have any issues with three car seats. We were so impressed we are thinking of getting one. Either that or a Commodore.

We currently have a CX7 as our big car and it does fit three car seats (my friend has one and recently had 2 toddlers in SnS Meridian's and a new born in a capsule on the back seat) but it's tight. I have a Mazda 3 and can already see that three car seats would be far too tight.

We are still considering getting the CX9 down the track so we have the extra seats as well as the bigger boot space so we may just upgrade my car to a Mazda 6 wagon but the Falcon is definitely on the maybe list now as my car