Why did the Third-estate propose a change in the Estate's-general's voting rules?

The system was that each estate took its own poll on a one-man-one-vote basis; the majority in each estate decided how that Estate should vote; and the final voting was by Estates. On tax questions, the Third Estate was bound to be outnumbered two to one, despiteits memebrs being far more numerous than those of the other two. Naturally, threfore, they wanted a single one-man-one-vote arrangement.