This sounds normal:

"Gas costs four bucks a gallon here."
"Four bucks is too much. Let's go somewhere else."

"The board is thirty-six inches long."
"Thirty-six inches is too short. We need a longer one."

"I bought two gallons of milk to make ice cream."
"Two gallons is not enough. We need at least four."

"Are" would sound odd in these sentences.

But once I put the word "people" in, the "are" sounds okay:

"Three people came to help us."
"Three people aren't enough to get the job done."

but

"Three people isn't enough to get the job done" is probably more common.

And once we get away from the number being the main focus, "are" becomes the only good version:

"Has everyone arrived for the meeting?"
"Three people aren't here yet because they are tied up in traffic."

while

"Three people isn't here yet" sounds horrible.