Cambridge police yesterday smashed a $50,000-a-month book-makers' ring that made this city its headquarters and principal area of operations. The gaming squad apprehended at least three men and promised more arrests today.

One police official said last night several of the group's 30-odd agents "undoubtedly" operated out of Harvard Square. He declined, however, to mention the location of the book-makers. The individual runners have not yet been apprehended, because they were known only by code numbers and nicknames.

The police culminated two-and-a-half months of investigation at 2:10 p.m. with a raid on the central headquarters at the third floor of a 861 Washington St. apartment house. In order to gain entry, one officer had to smash in the rear door of the flat with a sledge hammer.

Concealed Phones

Inside, they discovered gaming implements, an adding machine, four telephones, and dozens of envelopes with code numbers on them for runners. They also found a closet containing a trap door which concealed the phones and threw a switch disconnecting them in the event of a raid.

The police, after arresting the three men, again hooked up the phones and began taking bets. One member of the gaming squad estimated they were phoned in at the rate of ten a minute.