I can get two child seats for a 7yr old and a 4 yr old in the back of the LC but you could only ever fit a tiny booster seat between them.
Someone else made the point that with the amount of clobber you will need for the littlest one the Leon becomes impractical for any stay away journey in any event.
I've been looking at the pictures of the Altea XL and it looks like it might fit the bill, but still be fun to drive with the right engine. When I drove a basic Altea I was impressed with it's grunt and handling compared to our dodgy old Passat Estate. Unfortunately 3 kids is big estate or MPV time
But hey they're worth it?