@Julijana - Probably the easiest way for you to do this would be to use copy-and-paste to from the two boards with the content that you want to move into the third. And if you are wanting to move ALL of the board contents, this makes it even easier as you can open a board, use Ctrl + A to select all, then Ctrl + C to copy, and then go to the board where you want to move the content to and press Ctrl + V to paste.

If you don’t want to move everything, you could put a frame around the content you do want to move and copy the frame and then paste it into the other board.

You can find even more shortcuts to help select multiple board items in the Shortcuts and Hotkeys help article.

Aham,
Thank you very much. I guess I will use the frame option as I don't want to copy the information from the entire board!
Thanks again Robart, this has been very helpful!