I'm afraid that the best medical evidence disagrees with this; systematic meta-analyses of several large randomised controlled trials show that multivitamins have no effect in the prevention of major diseases (cardiovascular disease, cancer) or death; and that indiscriminate multivitamin supplementation may even cause some harms. http://annals.org/article.aspx?articleid=1789253

You can read more about the evidence around multivitamins, and the evidence of potential harms, here: https://www.sciencebasedmedicine.org/more-evidence-that-rout...

I'm well aware of this. This is advice for someone who knowingly keeps a nutrient-poor diet. (We're talking about an article on scurvy.) Doctors do advise vitamins in those cases.

Thankfully, I live in a country where all fruits are cheap and abundant, and multivitamins are expensive.

What would be the catch though? I'm also guessing that I won't reach 100% of all the health benefit even if I triple my multivitamin intake?

You won't get fiber and probably will have too much sugar and high glycemic index foods if you eat junk + a vitamin. But you can easily get all your vitamins and minerals with a pill.