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This problem is a good context for encouraging learners to work systematically. Then they will be able to convince someone else that they have found all the possibilities.
You could introduce the task by showing the class the interactivity and, without saying anything at all, roll the balls three or four times. Ask children what they notice and what they wonder, and invite them to talk to a partner.
Take some noticings and questions from the whole group, but try not to respond yourself. Instead, rely on other members of the group to comment or answer questions.
You can then clarify what is happening in the simulation, and ask how many different ways the balls could land. Look back at the results you have recorded so far in the interactivity, and invite learners to explore whether there are any other combinations. As they work in pairs on this (perhaps using coloured counters to represent the balls), listen out for those who are developing a system to find all possibilities. You may like to draw attention to those who have created a good recording system too.
In the plenary, you could ask particular pairs to share what they have been doing and you could facilitate a whole class discussion so that a conclusion is reached. You may wish to ask learners to record individual solutions on individual strips of paper, then you can pin some of these up on the board. Invite the group to reorder the solutions to reflect a pattern and use this to create any solutions that are not yet displayed. Emphasise that although they have worked together using one pattern, or one particular way of working systematically, there is no 'right' way to work systematically. There will be many different systems in the room and that should be celebrated.
If red landed in the middle, how could the blue and green fall?
Where else could red land if it wasn't in the middle?
Can you use this idea to find all the ways?
How will you remember the ways you have found so far?
Pupils would benefit from having three differently-coloured counters to use while tackling this problem. If they are finding it difficult to work systematically, you could offer them 6 Beads first, which might be a more familiar context.
Encourage children to use four balls/counters. If learners would like another context in which to practise working systematically, Inside Triangles would be an appropriate task.