Complex sentence: England retained the Ashes after the third Test match was drawn to give them a 2--0 lead in the series; since England already hold the Ashes, even if Australia won the next two Tests to draw the series, England would still retain the urn.
Split into simpler sentences: England retained the Ashes after the third Test match was drawn to give them a 2--0 lead in the series; since England already held the Ashes at the start of the series. Even had Australia won the next two Tests to draw the series, England would have still retained the urn.