1a and 2a are OK, but not great. A native might say them, but probably wouldn't write them.
3a is WRONG. If you use "for" or "since" to talk about duration, you HAVE TO use a perfect tense. It sounds very strange if you don't.
EDIT:
After further consideration, you COULD say something like "Tomorrow, I will sing for 20 minutes". There, the entire 20 minutes occur within "tomorrow". Similarly, you can do similar things with other tenses "I swim for 20 minutes every day" "When I was young, I went hiking for two days every week" "I went hiking for a month in 2015".
However, "20 years" is longer than "next week", so this interpretation wouldn't make sense, so 3a is simply wrong.