The Wallabies were in a position to level the series after taking an 18-point lead in the second Test at the MCG on Saturday.
But the home side were unable to seal the deal, allowing the Lions back into the game before fullback Hugo Keenan scored the match-winning try with 45 seconds left on the clock.
Wallabies flanker Carlo Tizzano appeared to have been cleaned out illegally with a shoulder to the neck in the build-up, but Keenan's try stood after a review.
It meant Australia suffered a 29-26 loss, with the Lions taking an unassailable 2-0 lead in the series ahead of the final Test in Sydney on Saturday night.
Whether the last-minute try should or should not have been awarded, Frost said there was a bigger picture surrounding their defeat.
"There were so many moments up until that where we could have capitalised," Frost said on Monday.
"We had a pretty decent lead in that first half, and we gave away a few tries.
"It doesn't always come down to the refs ... At the end of the day, you can't really rely right at the end for a decision there.
"If you're relying on that, you probably haven't done enough work throughout the game."
Tizzano has been accused by UK media and ex-players of staging, to try to win the penalty, but Frost defended his tough teammate and said the Wallabies didn't listen to the "outside noise".
The Wallabies will have pride, and unwanted history, to play for at Accor Stadium.
If they lose on Saturday, the tourists will have claimed their first three-Test series whitewash on Australian soil since 1904.
While starting prop James Slipper has become just the second Wallabies player to appear in two Lions series, Frost said the third Test presented another rare opportunity for all, with the tours held only every 12 years.
"There is something to lose. It's a Test match," Frost said.
"There's a bit of fire in the belly from the last couple of games ... And again, like it's a Lions series, it's our last time - unless you're James Slipper - to play against them."
Wallabies coach Joe Schmidt will be forced into changes after Harry Potter suffered a hamstring injury in front of his home crowd at the MCG.
Dylan Pietsch or Andrew Kellaway loom as options to fill that spot, joining Max Jorgensen in the starting wing positions.
"I've been there before and really feel for him," Jorgensen said of Potter.
"It's really tough to see him head out of camp and stay down in Melbourne.
Jorgensen agreed there was plenty of motivation for the Wallabies to fire up for the third Test.
"All the boys are pretty gutted after that result. We put in such a big effort - we were leading for 79 minutes of the game," the 20-year-old said.
"It's a tough pill to swallow. Everyone knows that, but obviously there's a big focus on this weekend now and winning this game this Saturday."
