Yes washing the bottom of your car will wash away salt in the winter. I'm too lazy to do it 3 times a month but you can if you want. You will never get rid of rust completely. Use the forum search box to see all the previous answers that were given.

I think you are overdoing it. If you don’t have any serious rust (not surface rust), just simply hose down car underneath every season. If you have serious rust, you should take care of it instead of washing it. 

Kwik Trip. A true midwest boy.

As a loyal Kwik trip customer, you are most likely in Wisconsin (In Iowa at some point they turn into Kwik Stars), which gets a heavy dousing of snow and road salt every winter.

If it is above freezing in the winter months, go ahead and wash it. But don't worry about doing it 3 times a month in the summer. I say once a month or so.

If you are trying to prevent rust along with washing your car you should also protect your paint by waxing it, clean your undercarriage every so often especially during/after winter. And keep your car dry.

Here's some information from Scotty on preventing rust on your car

https://www.youtube.com/watch?v=AxVfRuPyIQM

There's this rust prevention oil that some folks have applied to the undercoating of their cars (and businesses to their fleets, such as trucks) in the North and East. It's called "Fluid Film." (You wash it off in the spring). There are other such fluids, such as NH Oil and Krown. See how that is done, here. 

https://www.youtube.com/watch?v=QiIym5JiMOc