In #1 and #2,
percent is the grammatical subject. #1 is the standard because
money is the notional subject. Percentage itself is a rather vague amalgam of countability and uncountability, I suppose, so that the weight falls upon the notional subject. In #3,
two is the subject of course:
Ten percent of the children are hungry. (Ten percent are hungry. One-tenth are hungry.)
Ten percent of the water is polluted. (Ten percent is polluted. One-tenth is polluted.)
Two of the cars are in good condition (Two are in good condition)
The American Heritage Dictionary has this comment:
'•
Percent can take a singular or plural verb, depending on how
the quantity being described is viewed. Very often what determines the
form of the verb is the noun nearest to it. Thus one might say
Eighty percent of the legislators are going to vote against the bill or
Eighty percent of the legislature is set to vote the bill down. In the second sentence the group of legislators is considered as a body, not as individuals. When
percent is used without a following prepositional phrase, either a singular or plural verb is acceptable.'
However, I think that the last statement is misleading.
Ten percent is/are can both be acceptable, of course, but context will determine which is acceptable in that situation.