Use laptop as a monitor?

So…

I connected 2 monitors to a laptop. Can I still somehow set the laptop as a monitor? So that I have about 3 monitors.

Because now I can only use my two monitors, but not the screen of my laptop.

A laptop can't be used as a monitor. They have no inputs for picture signals, but can only supply an external monitor as a second.

If you expand your display as normal, then your laptop monitor should also be operated (usually even as the main display).

Check your setup under Settings / Display.

Yes, this is also possible if I only connect one monitor, but not if I connect two… If I connect two monitors, I can only display the laptop via either the laptop and one monitor, or both monitors. But don't stop over all 3

It depends on your built-in graphics card / graphics unit how many monitors the laptop can control.
It may be that your laptop can only operate two monitors at the same time.
With other laptops, however, you can easily connect two monitors and use your own monitor together with the laptop, so that you can work with three monitors.

Or do you want to connect the laptops to another device as a monitor?
This is usually not possible because most PCs / laptops have a video input. But one can e.g. Retrofit with capture card.

Can you somehow look up which graphics cards can and which not.

Yes, that's usually in the documentation of the respective graphics card.
So just google the name of the graphics card and you should find the appropriate documentation.

Is your laptop a bit older, any modern IGP can easily control three panels?