“Two wrongs don’t make a right, but three lefts do.”

Variant: Two rights don't equal a left.
Source: The BFG (1982)

Source: The Second Sin (1973), P. 49.

"Negroes Are Anti-Semitic Because They're Anti-White" http://www.nytimes.com/books/98/03/29/specials/baldwin-antisem.html in The New York Times (9 April 1967)
Context: It is true that two wrongs don't make a right, as we love to point out to the people we have wronged. But one wrong doesn't make a right, either. People who have been wronged will attempt to right the wrong; they would not be people if they didn't. They can rarely afford to be scrupulous about the means they will use. They will use such means as come to hand. Neither, in the main, will they distinguish one oppressor from another, nor see through to the root principle of their oppression.

The Contemporary Review
Context: AHow, then, can the rights of three men exceed the rights of two men? In what possible way can the rights of three men absorb the rights of two men, and make them as if they had never existed. Rights are not things which grow by using the multiplication table. here are two men. If there are such things as rights, these two men must evidently start with equal rights. How shall you, then, by multiplying one of the two, even a thousand times over, give him larger rights than the other, since each new unit that appears only brings with him his own rights; or how, by multiplying one of the units up to the point of exhausting the powers of the said multiplication table, shall you take from the other the rights with which he started?

"Former U.S. Ambassador To Russia On The Arrest Of Russian Opposition Leader" in NPR https://www.npr.org/2021/01/18/958120724/former-u-s-ambassador-to-russia-on-the-arrest-of-russian-opposition-leader (18 January 2021)

Book II, satire iii, line 50 (trans. Conington)
Satires (c. 35 BC and 30 BC)