it’s doable albeit not quite as straight forward as – presumably – you want to offer the same flavours for all three
reading your scenario, there is perhaps a way to an an option for this in the plugin at some point to make this simpler. alas , at the moment you would have to do wither of the following

option a): much simpler, but not really what you are asking for (but thought i’d mention it anyway).
– create 3 distinct menu items “3 balls”, “5 balls” , “7 balls”
– create all your flavours as ingredients
– create 3 custom groups, each having all the flavours but setting min/max requirements distinctly for each (which is why you need 3) and assign them to the menu items created above

option b) – i.e keeping one menu item with 3 sizes (3,5,7)
– create all your flavours as ingredients
– copy all these flavours 2x. So you end up with 3 times the same bunch of flavours (as every ingredient in a menu item must have it’s own id)
– create 3 custom groups, each having all the flavours but setting min/max requirements distinctly for each (which is why you need 3) and assign them to the 3,5,7 menu item above
– create 3 more custom groups,using “exlude” option to exclude as required (if all items from a group hve been excluded, the whole group will not show up anymore)

reading this through, i actually created this here with screenshots (as it might be be helpful)

it’s also a somewhat unique setup you are trying to achieve and just a way to make it work regardless.
it would be easier of course if one were to be able to just exclude a custom group from a size altogether or some such thing, instead of adding additional exclusion groups. alas , this is not implemented at the moment though might be one day

typically you do not have the same ingredients/topping (whatever you want to call it) multiple times for an item. furthermore they MUST have different id’s (reason for copying it)

>Its not the smartest way to handle my problem,
as i said, the easiest would be to have 3 different menu items. after all the plugin cannot possibly cater for every single unique way people want to do things. That said, it’s clearly still possible – as outlined