The 1-3-1 standard essay format is as follows: One introductory paragraph. One paragraph for each key point. (approximately 3 paragraphs) One summary paragraph. Search for "essay format" on the Internet. There are answers galore for you there.

for an argument, I would state my opinion in the first paragraph , followed by facts and figures that support my opinion in the following paragraphs, and concluded again with my opinion, which now has a list of evidences in the previous paragraphs.

I think it depends on the context, but in general I would introduce it at the start of what you're writing to create interest, then explain with an argument and then finish with the same original statement to ensure the point has come across.

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@Daniel When writing argumentative essays.