When concluding an essay, I wrote "what has been discussed are three major advantages of xxx". But I doubt if "what has been discussed is three major advantages of xxx" is more correct?
The following "Grammarphobia" blog post provides a summary of what some resources say about how to inflect verbs for grammatical number when what is the subject: When the complement was roses.
One of the more succinct [discussions of this problem] can be found in The American Heritage Dictionary of the English Language (5th ed.) [...] Here’s American Heritage (we’ve broken the usage note into smaller paragraphs):
“When what is the subject of a what-clause that is the subject of a main clause, there is greater variation in usage. When the verb of the what-clause and the complement of the main clause are both plural or both singular, the number of the verb of the main clause generally agrees with them.
“When the verb in the what-clause is singular and the complement in the main clause is plural, one finds both singular and plural verbs being used. Sentences similar to both of the following are found in respected writers: What drives me crazy is her frequent tantrums; What bothers him are the discrepancies in their accounts.
“Occasionally the choice of a singular or plural verb may be used to convey a difference in meaning. In the sentence What excite him most are money and power, the implication is that money and power are separable goals; in What excites him most is money and power, the implication is that money and power are inextricably bound together.”
So what’s known as “notional agreement,” a subject we’ve written about on our blog, plays a role here.
In your sentence, what is the subject of the clause "what has been discussed" (which has a singular verb), and the complement of the main clause is "three major advantages of xxx" (which is a plural noun phrase). So either "are" or "is" would be possible.