With Monkstown missing the chance to go top after losing 4-3 to Cork C of I, Lisnagarvey are level on points with Banbridge with a 2-0 win over UCD.
Instonians are still without a win at the bottom after losing 3-2 to Pembroke Wanderers at Shaw's Bridge.
In the other game, Glenanne defeated Leinster rivals Railway Union 5-4.
At Grange Road, goals from Jody Hosking and Ben Walker put Three Rock into a 2-0 interval lead before Johnny McKee replied for Banbridge after the break.
However, despite their first loss of the season, Banbridge do have a game in hand over Lisnagarvey and Monkstown.
Garvey scored a goal in each half against UCD to join Bann on 20 points with Ryan Getty finishing off a flowing move to break the deadlock in the 29th minute.
The Hillsborough side continued to force the pace in the second half and James Lorimer completed the scoring from a 42nd minute penalty-corner.
Just two points separate four teams at the top of the table with Cork C of I's win over Monkstown moving the Munster side level with their opponents and trailing them only on goal difference.
Instonians remain at the bottom of the table after a last minute goal denied them a draw against Pembroke Wanderers.
Stephen Kelso gave the Belfast side the lead in the the second minute but two goals in the space of four minutes close to half-time saw the Dublin side move into a 2-1 interval lead.
Jack Ryan equalised on the half-hour mark and Ryan McDonald made it 2-1 shortly afterwards but James Corry levelled with 10 minutes left when he scored from the penalty spot.
However, after some fine saves from home goalkeeper Sam Brown, Pembroke sealed the win when David Quinn netted from close-range in the final minute.
Glenanne's win over their Leinster rivals has moved them eight points clear of Instonians, who remain five points adrift of second-bottom Railway Union, with all three struggling sides having played the same number of matches.
Banbridge remain top of the Irish Hockey League but only on goal difference after losing 2-1 to Three Rock Rovers in Dublin on Saturday.