The 30-year-old, who is poised to make her Opals debut at the women's World Cup in Spain later this month, played crunch-time minutes during Seattle's 98-82 win over the Washington Mystics at Virginia's EagleBank Arena on Thursday (AEST).

Whitcomb notched six points - nailing two of her four three-point attempts down the stretch - two steals and one assist in 13 minutes on court.

The three-time WNBL All-Star with the Perth Lynx is now a WNBA champion after making her American league debut for the Storm at age 28 last season.

The American-born guard/forward, who received Australian citizenship earlier this year, was called into Australian squad on Monday to replace injured point guard Leilani Mitchell two weeks out from the FIBA tournament.

The victory catapulted the Storm to an unassailable 3-0 lead and series sweep in the best-of-five WNBA Finals.

It's the Western Conference franchise's third NBA championship after Australian Lauren Jackson led the side to titles in 2004 and 2010.

League MVP Breanna Stewart top-scored for the Storm with 30 points to earn the nod as the Finals' best player after averaging 25.6 points across the three games.

She scored 17 points in the first half as the Storm raced to a 47-30 lead.

Forward Natasha Howard added a career-high 29 points and 14 rebounds for the Storm.

WNBA legend Sue Bird, a teammate of Jackson's for Seattle's two previous championships, turned in a double-double with 10 points and 10 assists.

"Each (championship) is special in its own way, but this one is probably going to have a different meaning for me," the 37-year-old veteran point guard said.

"There is probably no comparison to be honest. I didn't know if I'd be playing at this point."

The Mystics, in their first Finals in franchise history, were far from disgraced rallying from 18 points down to trail 72-67 with 6:49 remaining.

But Seattle countered with eight consecutive points to pull away and put the game to bed.

Australian Associated Press