To use javascript you would use a Function node.
In that case I suggest also putting a value in the timeout field in the Join node. Maybe 2 seconds. Then if one of the values does not arrive, or maybe you start node-red in the middle of the sequence of three messages so one gets lost, then it will send them when the 2 seconds has elapsed. Then in the function node you can check that all three are there and decide what to do if one or more are missing.
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