Did you read the documentation for $items() which I linked above? There it describes how you can access the data of a specific node. If you follow that you can get the data of all 3 nodes.
I actually already explained above exactly why it is guaranteed that the Function-Node will execute after Command 1 …3 did finish. That is sadly all I can say about that. You can literally believe me or not.
If you do not believe me, use Merge-Nodes as @RicardoE105 said in his initial answer. You would have to use two of thoes. One to Merge the data of 1 & 2 and then a second to also merge the data of 3 in.