Varsågod / Snälla / Tack
Can you tell me the difference and if they are interchangeable?

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Hazal is correct. 
The usual terms when asking someone to sit down in Swedish is 'Varsågod och sitt' or the more casual 'Slå dig ner'.  
In English, the term 'Please take a seat' is commonly translated into 'Varsågod och sitt' in Swedish although the literal translation is 'You are welcome to take a seat'.

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I've seen the sentence Varsågod och sitt (please, take a seat)

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I'm just A2 in Swedish but as far as I can tell "tack" is  thanks, "snälla" means please and "varsågod" is you're welcome.