Yes, the sentence is correct and no, you can't remove the article. The sentence refers to three specific people who were struck by lightning and gives the additional information that two of them survived. If you remove the article, you are left with an ungrammatical sentence.
You could remove the article if you were talking about a regular/habitual occurrence. For example, "Two out of three people who are struck by lightning survive". You aren't talking about specific people or about a specific lightning strike but a fact based on previous events.
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