Trying to rove that the set of all context-free languages over a language Σ is closed under TRIPLE where TRIPLE (L1, L2, L3) = L1L2L3. Pretty much, TRIPLE, applied to three languages yield the concatnation of the three languages in order.
I am a little confused on where to start but my initial approach was to construct a grammar for the union of the set off all context-free languages and TRIPLE by taking all the rules from both grammars. But I am not sure how to go about this or if this is the right approach. Any help is appreciated.