I think this sentence uses the German imperative, being a polite command rather than a question.
Hypothetical situation: A man called Bob goes to a shop to buy some shoes which are very popular and are selling out quickly. When Bob asks the sales assistant for a pair of these shoes he is told, "There are currently none in stock due to their popularity however there will be a new delivery of these shoes in the next three days." The sales assistant then says, "Kommen Sie bitte in drei Tagen wieder" - "Please come back in three days".
Why is the pronoun Sie included? In the past lesson on Imperatives, the pronouns never follow after the imperative verb.
"Iss dein Gemüse." (not: "Iss du dein Gemüse.")
"Handelt schnell!" (not: "Handelt ihr schnell!")
I look forward to hearing from anyone. Any help will be appreciated.
The polite/formal Sie form of the imperative uses the pronoun: Gehen Sie bitte nach Hause; Bitte trinken Sie nich so viel Saft. Please note the use of the word "bitte" when giving a command in this form.
1st person plural also works this way: Triken wir jetzt diesen Bier
What you wrote means "She comes back again in three days, please". If you are talking in 2nd person (you to me) and in a formal way, you'd use Sie (always starts in capital) + verb in plural (as for wir, we, they). Sind Sie (Be you), not Ist sie. Sie essen vieles Essen (You eat a lo of food). And also this a command, so the verb goes first as in English. Do your homework! Sleep now! Etc.
Because it's in its Imperative Form . "Kommen Sie-" is the formal way of saying the command, "Come-".
"Kommen Sie bitte in drei Tagen wieder." -> "Please come again in three days." (formal)
"Komm (du) bitte in drei Tagen wieder." -> "Please come again in three days." (informal)