my battery and its reliability

Hello my friends mercedes W211 year 2002 E 220 cdi

I bought a new battery on January 31, 2020

and you should know that I don't drive or use my car often

once a week I drive I use the car and I travel 3 km (in the city)

and once 53 km round trip but in a highway

I don't use the car a lot

my problem according to the battery seller told me that you must at least run / start the engine you should not leave the car (battery) without turning / starting the car so that the battery does not run out energy (electricity) this information from the battery seller shocked me I don't need the car all the time once or twice a week no more than that

I haven't used the car for 03 days and yesterday I couldn't run the engine because of the b...