As long as it's not replaced too early, I believe 3DS numbers are a good bet, and 3DS + Wii-U an optimal result.
3DS numbers are not at all a "safe bet" if it's not replaced too early; unless Nintendo fuck up to a degree almost never before seen in this industry, Switch is going to blow passed 3DS numbers easily.
3DS is going to sell less than 80 million lifetime. Switch is tracking significantly ahead of it despite being $300 when 3DS was $170 at this point, and not having had a hardware revision yet. As long as it's not sabotaged, Switch will pass 100 million lifetime.