To figure out the basic X’s and O’s of hockey, one must understand that all aspects of the game are interrelated. What happens in one of the three zones of the ice — offensive, defensive or neutral — has a direct effect on what happens in the other two. For the Bruins, all good things start with the forecheck. And when the forecheck is lacking, they’re in trouble. In Game 2 of their first-round series against the Maple Leafs on Saturday, the Bruins were unable to establish their forecheck. Indeed, it was the Leafs who repeatedly came hard at the B’s with a physical forecheck. And it was the Leafs who won the game. But in Game 3 on Monday at the Air Canada Centre, the B’s again had their forecheck operating at full power. Line after line, they got pucks deep, they were first on the puck, they hit and won battles. If one player didn’t immediately gain control of the puck, he kept it alive until a supporting linemate swooped in to collect it. The B’s produced a bundle of good scoring chances, and they kept the Leafs backed up in their own zone, unable to generate the clean breakouts and speed up the ice they established in Game 2. When you talk about the Bruins forecheck, the conversation always turns to Milan Lucic — not the most talented player on this roster, but maybe the most important for establishing the physical tone the team needs to succeed. “He’s leading the way,” Brad Marchand said. “We’re very lucky to have him.” Lucic was at his best in Monday’s 5-2 victory, and the Bruins, who lead the best-of-seven series 2-1 going into tonight’s Game 4, need more of the same. The big winger had three assists in Game 3, giving him six for the series, and he has a plus-5 rating in three contests. The line of Lucic, David Krejci and Nathan Horton has piled up five goals and 12 assists so far. Don’t forget, Lucic struggled terribly throughout much of the abbreviated regular season, scoring just three goals in his final 31 games and suffering the ignominy of being a healthy scratch against Pittsburgh in late April. Now he’s the main focus in the Maple Leafs locker room. “He’s a guy skilled enough to play on their first line, who at the same time can play a strong, physical game,” Toronto defenseman Ryan O’Byrne said. “He’s a guy we’ve got to watch out for. We’ve got to make it tougher on him. He’s one of the better power forwards in the league. It’s a position, a role, that’s unique. There isn’t a whole bunch of players in the league who can fill that role of being a big body, being intimidating physically but at the same time carrying a pretty high skill set. “That line is good for them. They have a good combination of size on the wings, and Krejci is obviously a skilled, smart, speedy centerman. That line has had success for them for a number of years now. We have to make sure we’re limiting their chances and not giving them so much time and space.”