Final answer:
The sentence should have a comma before the conjunction 'and' to correctly separate two independent clauses in a compound sentence.
Step-by-step explanation:
The correct version of the sentence with one comma inserted is: "Arvind Mahankali finished third in the 2011 and 2012 Scripps National Spelling Bee competitions, and he finished first in 2013." This correction introduces a compound sentence, which is made up of two independent clauses connected by the conjunction 'and.'
The comma before the conjunction in a compound sentence is necessary for clarity and correctness. When writing or editing sentences, it is crucial to prevent common comma errors such as omitting necessary commas or inserting them where they are not needed. This helps maintain the intended meaning of the sentence and the flow of the text.