The Sharks used their superior speed to create odd-man rushes that paid off when Donskoi scored and then set up Marcus Sorensen in the opening four minutes of the second period to seize control of the game and series.
Eric Fehr and Tomas Hertl added goals later in the second to turn this all-California series into a rout. Logan Couture had a goal and two assists, Joe Pavelski, Evander Kane and Chris Tierney scored power-play goals in the third period, for the Sharks, who have outscored the Ducks 14-3 through the first three games of the first-round series.
Martin Jones made 45 saves, the most for a Sharks goalie in a regulation playoff game.
San Jose will look to complete the sweep in Game 4 at home Wednesday night. Only four NHL teams have lost a best-of-seven series after winning the first three games with the Sharks the last to do it in 2014 in the first round against Los Angeles.
Rickard Rakell scored a power-play goal but the Ducks once again looked a step slow against the speedier Sharks. Anaheim has scored three goals all series, including just one at even strength.
John Gibson allowed five goals on 24 shots being pulled after two periods.
The game turned early in the second after Brandon Montour fell near center ice, giving the Sharks a two on one. Donskoi converted on a give-and-go with Evander Kane and San Jose was off to the races.
Donskoi then set up the next goal on another odd-man rush when he fed Sorensen, who faked a forehand and then beat Gibson with a backhand on a similar move that led to goals for Kane and Couture in the first two games.
Jones then made a big stop on Rakell from the slot to preserve the two-goal lead and Fehr helped put the game away when he skated through two defenders and beat Gibson up high to make it 4-1.
That led to some predictable frustration for the Ducks and Nick Ritchie got into a scrum in front of the San Jose net with Dylan DeMelo, giving the Sharks a power play.
A slash by Francois Beauchemin gave San Jose a brief two-man advantage and Hertl converted on the power play after the first penalty ended to make it 5-1.
Pavelski, Kane and Tierney scored in the third against Ryan Miller.
Fresh off the two wins in Anaheim, the Sharks were greeted by a frenzied crowd that sparked them to a fast start. Mikkel Boedker got things started when he used his speed to get around Hampus Lindholm and feed an open Couture in the slot for the tap-in into the open net.
San Jose maintained the pressure but Gibson stopped Chris Tierney on a breakaway and the Ducks then killed a penalty. That seemed to turn things in Anaheim's favor as Timo Meier was sent to the box for San Jose and the Ducks capitalized.
Rakell, held to no points and two shots the first two games after scoring 34 goals in the regular season, took a pass in the faceoff circle from Brandon Montour and sent a one-timer off the far post and into the net for the equalizer.
NOTES: D Andy Welinski was back in the lineup for the Ducks after being scratched in Game 2 for Kevin Bieksa. ... F Jason Chimera got his first action of the series for Anaheim in place of Antoine Vermette. ... Sharks C Joe Thornton, who has been out since Jan. 23 with a knee injury, took part in pregame warmups once again but isn't ready to play.
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