Only the third part is listed on hkmdb, but according to my taiwanese friend there are 3 parts and a spinoff.

Here are the links from his database
???
http://ringo.shockstar.com.tw/main.php? ... 6W%ACM%B5e
???2-?????
http://ringo.shockstar.com.tw/main.php? ... 6W%ACM%B5e
???3-???(already on hkmdb)
http://ringo.shockstar.com.tw/main.php? ... 6W%ACM%B5e
And ????????? aka War Dogs
http://ringo.shockstar.com.tw/main.php? ... 6W%ACM%B5e

Added the first. The 2nd was in our DB, but under "Naughty Cadets on Patrol"

I think Naughty Cadets on Patrol is another film.

if you compare the posters of "Naughty Cadets on Patrol" and "Dai Tou Bing 2" you will find that at least 2 of the bust photos are the same.

According to this, it is the Hong Kong version(s) of the title
????2 ?????????????????????2 ????????????
http://zh.wikipedia.org/zh-hk/%E6%9C%B1 ... 6%E5%B9%B3

Also found this, 4 films were directed by Chu Yen-Ping and a couple more in the series.
http://zh.wikipedia.org/zh-hk/%E5%A4%A7 ... B%E5%88%97
????1987??
???2 ??????1987??
???3 ????1987??
???????????1991??
???????????1993??
??????????????1998??

added "The War Dogs"