Origin and usage of "a shambles"

"Shambles" is one of the few singular nouns in English that blatantly resembles a plural noun. What is the origin of "a shambles"? Why do we really need to prefix an "a" in front of "shambles"? Which of the following is perfect?

• The country's economy is in shambles.
• The country's economy is in a shambles.
• The country's economy is a shambles.

The weird usage seems to have developed in the same way as "a headquarters". Consider the following Ngram:

The original phrase seems to have been "shambles" or "the shambles", almost always plural. It meant "a place for slaughtering animals", but later came to be also used for scenes of carnage, scenes of great disorder, and certain dangerous shoals at sea.

Etymonline says that "shambles" originally meant "meat market", having evolved from the word schamil meaning "table or stall for vending". I assume that it naturally started out as a plural, because there would be several butcher's stalls at a meat market.

However, once "shambles" came to mean "a scene of great disorder or carnage", the things it referred to were more naturally singular, and so "shambles" slowly came to be used as a singular.

"in a shambles" or "a shambles" would be correct. The only form that the last one would work for is "shambolic".

Given the origin, it actually does make sense, because the object is in [or just is ] a place of chaos and disorder - one of many such places. But it seems that an economy should be a shamble. The s is not a pluralisation, but a part of the word.

In York, UK, The Shambles is still a street, and a popular one for tourist shops.