Thanks. We are allowed 10kg in cabin (luggage and handbag combined) so weight will not be an issue, just that little extra in measurement totals (W + L + D).
I just don't want to pay for three cabin bags to be checked at the airport. I'm going to get our son to wait for ten minutes in the car park when he drops us off and if we have to check any bags, I'll run out and throw the contents of three cabin bags into one suitcase. I don't mind paying a little extra per bag but I read somewhere that last minute checking in of bags can be $40 per bag. I'll check again tomorrow to see where I read that, but I'm not going to pay $40 x 3 (and the same on our return) when I can transfer to one bag and pay once.
Anyway, thanks for the quick response.