On proving NP-completeness by reduction to 3-partition problem
No, that doesn't prove NP-completeness.


The thing you describe isn't a reduction.  It must be that the instance of&nbsp;$X$ has a solution if, and only if, the instance of 3-partition has a solution.
Reducing $X$ to 3-partition just proves that $X$&nbsp;is in&nbsp;NP.  To prove completeness, you need to reduce an NP-complete problem to&nbsp;$X$.


I suggest that you revise the basic concepts from your class notes, textbook or online resources.