STONEHAM – A lethargic third period made it far more difficult than it should have been but the Marblehead hockey team took the first step toward defending its Division Three North title, knocking off Latin Academy, 5-3, Sunday night at Stoneham Arena.After building a 4-0 lead through two periods, the No. 2 Headers (16-4-3) needed Ryan Dempsey’s empty-netter with 8.6 seconds remaining to seal the deal. Marblehead next meets No. 3 Dracut in the second half of Thursday night’s Sectional semifinal doubleheader at the Chelmsford Forum (8:10 p.m.).”Everything we talked about and focused on we did exactly the opposite of in the third period,” said Marblehead coach Bob Jackson. “Give Latin Academy credit – they played hard, kept working and didn’t play the scoreboard. They went shift to shift. It’s a good lesson for us. We’ll learn from it and go forward.”The Headers held LA without a shot on net in the first period – goalie Tony Cuzner’s first touch came with 4:04 left in the period when he covered a crossing pass through the crease – but didn’t get on the board until 1:34 remained. Diminutive Dragons goalie Edward Meagher (23 saves) turned back a couple of shots during a prolonged Marblehead flurry before freshman Tom Koopman pounced on a rebound and swept home a backhander from the right slot at 13:26.The Headers picked up the pace considerably in the second period, triggered by Dempsey’s end-to-end dash and top-shelf finish over Meagher’s blocker that made it 2-0 at the 6:27 mark. With 7:53 left in the period, defenseman Jake Kulevich carried deep into the LA zone before dropping a pass back to Austin Haley, who ripped a one-timer past Meagher from the right circle for a 3-0 bulge. Dempsey’s doorstep finish with 1:08 left in the period sent the Headers into the room after two periods with a seemingly comfortable 4-0 lead.”We talked about using our size and speed against them and we did that in the second period. The second period was solid,” Jackson said.The Dragons gained life on defenseman Kevin Sweeney’s power play goal at 1:49 of the third period. Remarkably, defenseman Thomas Hynes’ goal 42 seconds later came on LA’s first even-strength shot on goal in the game and cut Marblehead’s lead to 4-2. Things got dicey when sophomore Chris Kelly connected from the left circle with 4:03 left but the Headers regrouped and closed it out as Dempsey completed his hat trick.