Its a real to grow 20-30 grams on plant in 3,5 lit...

You can get 20-30 grams dry in a 16oz cup

Only if you have perfect environment and lights are good if not then I'd say a big no sorry

It's real. There's some people uploading diaries on that. There's a solo cup that nailed 25 grams...Search for that category: solo cup

This question isnt answerable.. you dont give any information about what medium you are going to use. We have no idea what plant you are going to grow and what light you are using. Is this your first grow? Unable to give a proper answer... Giving us the potsize is only one of the things that will influence your end result..

Are you talking about dry weight ? It seems a lot. Maybe with dense buds.

If you are very, very lucky, but 3.5 litre pots are way too small, in practice, 11 litres is the absolute minimum I would consider for autos or 20 litres minimum for photoperiod plants. In 3.5 litre pots you would be more likely to harvest 5-15 grams.