Have 3 apx 100 to develop in rodinal. The tank takes 1000ml, the amount of developer for the three films is 20ml? (rodinal 1:50)?
Or this is just the amount of developer for one film in a 1L tank? As if I develop one roll in a 500ml tank, rodinal 1:50, the amount is 10ml for one film only?
My box of 0,5 liter (Agfaphoto) Rodinal says it can be used to develop up to 80 rolls, so even by the original recommendation, 20 ml will be enough for the three rolls. Many of us have different personal rules for minimum developer stock/concentrate amount. 5 ml is fine in my experience with Rodinal, others would say less and a good number of people will recommend even 10 ml.
Actually, Agfa recommend 10mL concentrate per roll. Plenty of people on the internet think that 5mL is totally fine, but there are demonstrations of it exhausting slightly with negatives that are heavily exposed, i.e. shot in high key. For normally-exposed or under-exposed / pushed negatives, 5mL shouldn't have any problem at all.