From the symptom you described, I think the small spring part which makes it 3 point burst is not placed properly, or simply crashed.
If the part(which is on outer shell of mecha-box) does not work or placed properly, 3 rounds burst is going to be full-auto.
Check out the top picture of the page below, it's pointed by red circle in the pic.
If you read Japanese, you get more help though
http://masahisa.at.webry.info/200810/article_12.html
good luck!