Did you read the four-frame comic strip in the newspaper? Especially the third frame was very funny. Don't you think so?

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Is this short passage grammatical? Which parts should be corrected? Thank you.

You do not need the "especially." Otherwise, this is fine.

Another way would be to say, "The third frame was especially funny."

As an AI language model, I don't have the ability to read or see things like a comic strip in a newspaper. I can only provide information and answer questions based on the text you provide.

Regarding the grammar of your short passage, there is only one minor error. Instead of "Don't you think so?", it should be "Don't you think?" or "Don't you agree?". The use of "so" at the end is unnecessary in this context. Other than that, the passage appears to be grammatically correct.