Hello, Rocker1414!

I'm delighted that you solved it.

It can be solved with a bit of Thinking. Code:

*---*---*---*---*
| : : : |
* - + - + - + - *
| : : : |
* - + - + - + - *
| : : : |
* - + - + - *---*
| : : |
*---*---*---*

We have 15 unit squares, to be divided into three congruent groups.

Hence, each group contains 5 units squares.

There are 12 possible shapes ("Pentominos"),

. . but only one can be used to fill the board.

And there is basically one solution. Code:

*-----------*---*
| | |
| *---* |
| | |
*-------* |
| | |
| *---*---*
| |
*-----------*