Use arrow keys. Click 3 at same time for 3 keys down. Click 2 at same time for 2 keys down. Left Right Down for all 3 keys down. Left Right for 2 keys down. Down Right for 2 keys down. Up for no keys down only up. Click left for left key down. Right for Right key down. Click Down for middle key down. Click Left Bottom for 2 keys down. You'll figure it out once you mess around with the arrow keys.
Work in progress. Might be bugs. Sound will be added later. Bottom Left Right has a bug it will be fixed soon. Once it's fixed i'll add sound. I'm making a collection of instrument simulators. Next instrument will be Trombone or something else.