What's the difference between the pronouns "de", "den" and "det"?
I was trying to understand their exact meaning but I need some more help.

1) So, "de" means "they" and it's used while talking about people? E.g.:
De (Åke och Pia) har många böcker.

2) "Den" means also "they". I found this example here:
Den (Affären) har många kunder.
So, does it mean that I should use the pronoun "den" if I mean people in the collective meaning like "they in the company"?

3) What about "det"?

Question

What's the difference between the pronouns "de", "den" and "det"?
I was trying to understand their exact meaning but I need some more help.

1) So, "de" means "they" and it's used while talking about people? E.g.:
De (Åke och Pia) har många böcker.

2) "Den" means also "they". I found this example here:
Den (Affären) har många kunder. 
So, does it mean that I should use the pronoun "den" if I mean people in the collective meaning like "they in the company"?

3) What about "det"?

Answer

"De" means 'they' as mentioned.
"Den and det" on the other hand means 'it' and are used depending on if the noun is a "en" or "ett" word.
"Dem" means 'them', but can often also be used as 'they'.

Examples:
De är hemma = They are at home.
Den här saken är min. = This thing is mine.
Det är snart jul. = It's soon Christmas.
Kan du hämta dem? = Can you pick them up?

Answer

you use the pronoun "den" when referring to a word with the "en" article and "det" when it has an "ett" article.