Is it necessary to change the 12V battery after 3 years or can it still be extended by checking the condition?
It's necessary by the service "law". Meaning you loose your hv battery guarantee if they don't do it.
I had no guarantee on my HV battery and I change the 12v battery after about six years, but I do check it every few months. IMO changing the 12v battery every three years is expensive overkill, but I guess if you sleep better at night, that's something to consider.
2014 EV Coupe 22,500 miles
2014 EV Cabriolet 24,000 miles
2015 EV Coupe 22 000 miles
My 12v battery failed on me after about 4 years. Was rather inconvenient, as I had to replan a whole day and move all appointments. So I rather have it replaced by the service than by surprise.
But I also do think smart should simply use 12 v batteries which are meant for EVs. This way they would last at least as long as the HV battery and they wouldn't have to replace them. Right now they use the same battery as they used in the petrol cars. Which simply aren't made for the EV use-case.