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YOUR ANSWER: Three is the subject and equals is the verb.
You are correct. The sentence "Three equals two plus one" is about the quantity three, so three is the subject. The verb equals is the same type of verb that we found in the sentence "All women are poor drivers." That is, it functions chiefly as a connecting link between the subject, three, and the expression two plus one, which completes the statement made by the sentence.
Now, let's take our sentence "Three equals two plus one" and write it the more usual mathematical notation. It then becomes
3 = 2 + 1.
We can still recognize the parts of the sentence in this form, which is called an equation and is probably the most common kind of mathematical sentence. This mathematical sentence is called equation, of course, because it says that one quantity "is equal to" or "equals" the two other quantities.
Let us now consider another equation, or mathematical sentence.
2 + 1 = 3
In this sentence, what are the subject and verb?