First, we find the perimeter of each board by adding their sides
![P=6+6+6+6=24ft](https://img.qammunity.org/2023/formulas/mathematics/college/967lu76huzll2fm9bnrkc1pxiznwu78ijx.png)
The perimeter of each board is 24 feet long.
Then,
![24*3=72ft](https://img.qammunity.org/2023/formulas/mathematics/college/r77nkn9cu0bf2l5augtydwzny3rl9u2uq0.png)
The teacher would need 72 feet to decorate all three boards.
If each roll has 20 feet, then there would be needed 4 rolls in total to half enough to decorate all three boards.
Hence, the number of rolls is 4.