deposit is 2x rental price, not 3x. (yes, it says 3times in the t&c, but it is not the case)

unless it is a car from the X-categories (XTAR, XDAR, XFAR), then the deposit can be higher.

However, this is the rule for "normal rentals".
3 weeks is almost longterm, Sixt probably has different rules for that then.

btw. the deposit is not for damages etc. it is for returning the car late, not fueling up and things like that.
the bill for damages comes separately.