What is the case and syntax of trhe word feet in the sentence "The center of a tennis net is three feet long."

In this sentence, "feet" is an adjective modifying the noun "long."

feet is part of a dimensional adjective "three feet long". Feet modifies three, which together modifies long. The usage is as a predicate adjective.

Three feet long is the center.
The three feet long center is there.
Is the three feet long center there?

feet is part of a dimensional adjective "three feet long". Feet modifies three, which together modifies long. The usage is as a predicate adjective.

Three feet long is the center.
The three feet long center is there.
Is the three feet long center there?