Game 6 is Friday night in Washington, where the Wizards easily won their two games. If the Celtics win, they would move on to host the Cleveland Cavaliers to open the Eastern Conference finals.
Game 7 would be in Boston on Monday.
Boston led by as many as 26 points, negating Washington's physicality by spreading the floor and knocking down 16 3-pointers.
John Wall had 21 for the Wizards, who haven't won a playoff game in Boston since 1982.
The Wizards scored the first four points of game, but that was followed by 16-0 run by the Celtics.
It marked the fourth time in the series that one team has had a run of at least 16-0, including three times in the first quarter. Washington had a 16-0 spurt to start Game 1, a 22-0 run in Game 3 and 26-0 burst in Game 4.
During Boston's first-quarter spurt Wednesday, the Wizards missed eight straight shots before it was ended by Otto Porter's basket.
Boston also did a good job of sharing the ball, getting assists on 12 of 13 baskets in the opening 12 minutes. Thomas had five assists, more than Washington's entire team (four) in the period.
The Wizards responded with more energy after halftime, but each of their mini-runs were quickly beaten back by Boston, which continued to share the ball and take advantage of their fast break opportunities.
The Celtics' lead grew to 26 points in the fourth quarter.
Wizards: Fell to 2-9 all-time on the road in the playoffs vs. Celtics. Had 13 turnovers, leading to 23 Boston points. Shot just 24 percent from the 3-point line (7 of 29).
Celtics: Shot 53 percent (46 of 87) from the field for the game, their highest of the postseason. .. Wednesday marked only the third time Bradley has scored 20 or more points in a playoff game. The first two both came against Chicago in the first round. Shot 58 percent from the field (25 of 43) in the first half.
Boston 3, Washington 2
Sunday, April 30: Boston 123, Washington 111
Tuesday, May 2: Boston 129, Washington 119, OT
Thursday, May 4: Washington 116, Boston 89
Sunday, May 7: Washington 121, Boston 102
Wednesday, May 10: Boston 123, Washington 101
Friday, May 12: Boston at Washington, 5 p.m. PST
***Monday, May 15: Washington at Boston, 5 p.m. (if necessary)