Fill empty space in parent in UI Layout

See the Image for an easy description of the problem:

86515-mspaint-2017-01-23-04-18-19.png

I have a horizontal Panel with 3 elements in it.

  • The first Panel should be aligned to
    the left side and have a fixed width.

  • The last Panel should be aligned to
    the right side and have a fixed
    width.

  • The middel Panel should fill
    the remaining space in the center

.
I tried a long time but unfortunately I can’t get it to work.
I have a hard time beleiving that something so simple is hard (or maybe even impossible?) in unitys UI System.

This is how I have done :