勞工相關法律問題，被老闆克扣佣金是否可以追訴。

You may want to know whether or not the commission rate is clearly stated on the employee contract. If the commission rate is clearly stated, then no doubt you have sufficient evident to sue or report the case.
Or else, you may want to consider one thing. How are you going to prove the commission rate? Certainly, commission offer by oral agreement at the time you were hired is a valid agreement, but how are you going to prove it if your employee denied it. Also, you have accepted the commissions for 3 consecutive years, which, by law you are accepting the rate employee offered to you. According to imply rule, if, for your case, the commission calculation and/or method is not mentioned or stated on the contract, then the historical payment offer and acceptance will be the contract binding calculation and/or method for your commission.
If you are able to grab more employees to stand up and report the case, then you still stand a chance to win it.
Surely employees rule and regulation do protect you from getting commission from employee. However, there is no limitation on how the commission is calculated.

工資是指僱主支付僱員所有能以金錢計算的酬金﹑入息﹑佣金﹑津貼﹑小費﹑服務費及勤工獎，不論其名稱為何或以何種方式計算。
年終酬金、產假薪酬、遣散費、長期服務金、疾津貼、假日薪酬、年假薪酬及代通知金，都根據上述工資的定義計算。

任何查詢，可致電查詢熱線2717 1771（此熱線由「1823政府熱線」接聽），或瀏覽勞工處網頁http://www.labour.gov.hk。