Nutritional facts on Vitamin supplements - What is meant by serving size

I'd like to understand the nutritional facts before purchasing the vitamin product.

It's a very basic question.

The nutritional facts on vitamin product,
Serving size 3 caplets

Amount per serving,
Vitamin D3 600 IU
Calcium 1000 mg
Zinc 25 mg

Can y'all tell me what is meant by serving size and amount per serving?

Do I need to eat 3 caplets to get the amount of nutrition listed?

Or I should I just eat 1 caplets to get all nutritions listed?

Also, one more question,
Zinc 25 mg daily for a 18 year old enough or too much a day? I also workout whenever I can.

Thanks.

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