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