Wand wash or what I guess is called, "Do it Yourself".
Per the link below, here in Alberta, we have heated buildings, (not air conditioned), that consist of a number of wash bays where one drives your vehicle inside, puts coins or a credit card in some sort of money box with a knob or buttons on it. Depending upon the button pushed, high pressure water and perhaps a soap mixture sprays out of a "wand" kind of Star Wars like and in time, the 3 gets cleaner - how much depends upon the effort you put into it.
Pushing more buttons gets clear water or wax etc, and one button gets you a wash brush that a soapy water solution gushes out.
My cost is around ten dollars.
Given that it is near winter here perhaps eight months a year, and cold for at least three months, leaving the 3 dirty two thirds of the year really is not an option. I washed my 3 last week when it warmed up to -20C; -30C is a bit too cold, but I do when I have to as sometimes one has to get the frozen ice out of the wheel wheels in-case the air suspension drops and hence the wheels will not rotate as they are stuck in the now smaller wheel wells.
http://hughescarwash.ca/gas-car-wash/