Two lights are better, but if you use thee pendants, then position the two stools should be in the middle of the center and end lights.Although they will look very huge. Pictures would help.

Splashbacks are all about protecting the walls behind worktops, hobs and sinks from splashes. A splashback can make a small kitchen feel larger, and brighten any space. You can Choose splashbacks from plain mirror, antiqued finishes or colored versions. Pick good looking but easy care splashbacks. You can match splashbacks and worktops. You can also choose Image on glass NZ service, it will freshen your kitchen up with quirky.

400mm sounds good! Did you go for a smaller pendant?

I love three in ones because if one bulb explodes, you've usually got a fair few backups before you have to completely fix the bulb (but that's just the laziness in me coming out). In my opinion, I don't think you need a demisting mirror because that's what the fan is for in the three in one.