I have three wires from the lightbox: one black (hot), one blue (neutral?) and one yellow&green (ground). (I live in the Czech Republic right now and that's the wiring the place comes with...)

The light fixure I bought, however, needs two black wires, one blue and one yellow&green. (The light fixture has three bulbs in a row - the two at the ends need the one black wire, the one in the middle needs the second black wire.)

Can I just attach two wires from the one black one, cap it, thus creating two black wires which I can then attach to the fixture, or how do I solve this?

Thanks to anyone that can help!!

This is what I understand about that, though I have never worked on Czech wiring.

I think you have the incoming color code right: black (or brown) is the hot wire, blue is the neutral, and the green/yellow is the ground.

I believe that fixture is set up similar to some USA ones -- designed so you can have 2 switches to turn each set of bulbs on separately, or all of them at once if you want more light.

So if you are wiring it to use only 1 switch for it, you would indeed wire the incoming black wire to both of the black wires from the fixture. And then the blue to blue and green/yellow to green/yellow.

Try that, and see if it works the way you want.