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.

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.

The number you need for adequate lighting is dependant on the light fitting, its brightness and the angle the light will cover. The physical size of the fittings will also affect the number that looks best. Two very small fittings may give adequate light but look lost, while three larger ones could seem crowded. You may even find some very small fittings that work well as a group of six. Take into account the width of the surface as well as the length so you can ensure it is all well lit. You may be able to change the brightness of the lamp in the fittings to give a good work light in the number of fittings that look best. Avoid creating strong bright/dark contrast areas on a work surface. More overlapping less-bright light is better than few brighter ones that leave dark patches between them. Also consider whether there are other elements in the space that the group of light fittings could relate to, whether window mullions, cupboard doors, seating etc. If something else is creating a strong rhythm, decide whether you want to echo, or offset that or have the lighting as a background element that doesn't have a strong relationship to it.