If the individual parts are to be controlled individually, then you actually need 3x the starter set.

Can the "basic element" with a length of 2m also be used as an extension? (so two basic elements =4m)

Hi, you can also group the three together in the Hue app and then control them together. The app is very flexible, so you can create any combination of your lights. But of course you have to have a power supply for each part you cut. Otherwise, of course, nothing works ;) Alternatively, you can wire the individual parts of a cabinet light "by hand" so that you can place them in the right places. The guarantee will probably really be gone, but Galaxus can answer that best.

Have fun :) I think Hue is cool!