A five-seater S Max would be my choice.

Why not a Galaxy?, it is after all the same car but with more headroom. Actually any 'full sized' 7 seat MPV would suit with the 3rd row seats either removed or folded into the floor.

It depends how big the three adults are.

Three relatively slim, relatively short adults will be perfectly comfortable across the back seat of an Astra, but that's not going to be very comfortable for three rugby prop forwards!

Assuming the people only want to sit in two rows, if they are too big to be comfortable in a big car like a Superb, Insignia etc, there isn't much wider that it can get without being a van - a MPV or SUV may offer a bit more shoulder room, but the overall seat base is about the same width.

Even then, three people across the front of a Transit is not exactly spacious for the middle passenger, though they do have to allow a bit more width for the driver so they can use the steering wheel and gearstick. The big 'luxury' van based MPVs, used by hotels/airlines as posh taxis, such as the Merc V Class, VW Caravelle etc. only have two seats across the width as three full size executive seats wouldn't fit.

The best compromise is probably going to be a full size 7 seater like an S-Max or Galaxy, and just leave the rear two seats folded all the time to have a decent sized boot.