The New York Islanders are still alive in a bid for a third straight Stanley Cup. They do not have time to reflect on their good fortune in maintaining that status, however, because the next challenger, the New York Rangers, will be at Nassau Coliseum tonight to begin a best-of-seven Patrick Division final series.

The other divisional finals also start tonight, with Quebec at Boston, Chicago at St. Louis and Los Angeles at Vancouver.

The Islanders trailed Pittsburgh, 3-1, with six minutes left in the decisive fifth game of their first-round series Tuesday night. Then Mike McEwen scored a power play goal, John Tonelli tied it with 2:21 left and Tonelli won it in overtime.

It was the first time since the 1979 semifinals, when they lost to the Rangers, that the Islanders had been in a must-win situation. They had swept to the Cup the last two years without ever going the limit in any series.

It was hard to say who was happier, Quebec or Boston, after Dale Hunter's overtime goal gave the Nordiques a 3-2 victory in Montreal Tuesday. Quebec's success makes the Bruins a genuine Cup contender, since they had the NHL's fourth-best regular-season record while going 0-7-1 against the Canadiens, a jinx club through the years. Boston has lost 14 straight playoff series to Montreal. The Bruins won the season series from the Islanders, however, 2-1.

Los Angeles' 7-4 triumph over Edmonton guaranteed that the Campbell Conference representative in the Stanley Cup final will have a sub-.500 record. It was a calculated risk at realignment time that the final might prove anticlimactic, but NHL officials were counting on Edmonton or Minnesota to put up a respectable showing.

Both finished first in their divisions during the regular season, only to be ousted by fourth-place clubs. Chicago tripped the North Stars in four games and the Oilers fell in five to Los Angeles, the NHL's No. 17 team that finished behind Washington in overall points.

Play continues Friday at the same sites as tonight, then shifts for contests Sunday and Monday. If further games are necessary, they are scheduled Wednesday, Friday and Sunday.

Here is a rundown on each series: N.Y. Rangers at N.Y. Islanders

This is the fourth playoff series between bitter rivals. Islanders achieved instant respectability with first-round upset of Rangers in 1975. Rangers won 1979 semifinal in six games, pinning "choke" label on Islanders, who removed it by winning Cup in 1980 and 1981, sweeping Rangers in four-game semifinal a year ago . . . Islanders won six of eight from Rangers this year, including 7-3 rout at Madison Square Garden last week of regular season . . . Rangers lost first game to Philadelphia, then captured three in row to earn extra rest while Islanders struggled with Pittsburgh . . . Clark Gillies has blossomed as playoff scorer for Islanders, with four goals . . . Ron Duguay has four for Rangers, despite ejection from one contest after 104 seconds.

Quebec at Boston

Teams divided weird regular-season series in which each scored three victories on road, one at home. Nordiques scored 25 goals in four Boston games, Bruins 21 in four Quebec contests . . . Bruins have stuck with rookie goalie Mike Moffat, who had played only two NHL games before drawing starting assignment against Buffalo. He allowed Sabres 11 goals in four games . . . Nordiques received outstanding goaltending from Dan Bouchard as they upset Montreal in emotional five-game series . . . Quebec won that one despite loss of its leading scorer, Peter Stastny, with kidney injury in first game . . . Bruins, who last won cup in 1972, were beaten by Montreal in 1977 and 1978 finals.

Chicago at St. Louis

Teams finished with same number of points, 72, in regular season, but Blues earned home ice on basis of more victories . . . Blues took season series, 4-2-1, although Hawks won in St. Louis, 7-4, on last day of regular campaign . . . Hawks upset Minnesota in four games, while Blues did same to Winnipeg as Brian Sutter scored seven goals . . . Blues of 1968 were last sub-.500 finalists, losing to Montreal in four straight . . . Hawks snapped Stanley Cup record 16-game losing streak with three-game sweep of Blues in first round in 1980 . . . Chicago won teams' only other playoff meeting in 1973 quarterfinal, 4-1.

Los Angeles at Vancouver

Canucks are unbeaten in last 12 games, including three-game sweep of Calgary in first round of playoffs . . . Coach Harry Neale has sat out eight of them and must miss two more before he can go back behind bench. Neale turned reins over to Roger Neilson after NHL suspended him for fighting with fans in Quebec March 20 . . . Los Angeles Coach Don Perry was suspended for six games earlier as result of Paul Mulvey incident . . . Kings took season series, 3-2-3, although suffering 6-0 and 7-4 losses to Canucks on final weekend of regular season . . . Charlie Simmer and Marcel Dionne each scored four goals as Kings shocked Edmonton.