Each of the competitors — Company X, Company Y, & Company Z

The issue here is subject-verb agreement - specifically the issue of singular versus plural. If the subject is singular, then the verb must be singular. If the subject is plural, then the verb must be plural. In the original sentence, the subject and verb are "Each ... were." Remember that each is singular; don't be distracted by the plural nouns in intervening phrases - they are not the subject. This pairing of "Each" with "were" violates subject-verb agreement, so eliminate (A).

Note that you can vertically scan the first few words and last word of each answer choice and quickly do some further elimination.

(B) fixes the problem by simply changing the verb to "was." Now there is a singular subject and a singular verb. Suspect you've found the correct answer, but check the others to be sure.

(C) fixes the agreement problem by using "Company X, Company Y, and Company Z ... were," but this choice adds the awkward phrase "each of them being competitors." Remember that -ing constructions are often wrong on GMAT Sentence Correction questions.

(D) fails to fix the problem, using aplural subject and singular verb: "Company X, Company Y, and Company Z ... was."

(E) incorrectly changes the subject and verb to "competitors ... was each.

(B) is the only choice that fixes the problem without introducing new problems. Therefore, (B) is correct.

1/ Each of Noun : is always a Singular noun phrase => was => (A, C: out)
2/ Being : used for a passive + gerund form. Also, "being" is often fishy in GMAT. Be careful with it! -> C, D: out
3/ competitors: Plural Noun -> were -> E: out

=> IMO: B