Belfast High march on after winning away from home for the second time.
They will now entertain Sullivan in the fourth round after Larne Grammar found the rather robust Belfast High pack too difficult to handle.
The Belfast boys were 7-0 ahead at half-time with their pack gaining most of the first-half possession. The try came from good forward work before Matthew Huntley broke off to score and Robbie Fryars converted.
Eight minutes into the second period sustained pressure and another forward drive led to a ruck and Ryan McGonigle was driven over for an unconverted try.
Larne came back and within four minutes out-half Aaron McBride touched down for Philip Andrews to convert.
With 10 minutes remaining hooker Aaron Blair put the issue beyond doubt with a well taken try which Fryars again converted.
Belfast High coach Derek Wilson was pleased with his team's efforts.
“We exerted plenty of pressure in the first-half and should probably have been further in front at the interval,” he said.
“In the second-half Larne started to put their game together, but we defended well.
“We have now been drawn against Sullivan, a team we haven't played yet, but it will be uplifting to be at home'.