It may not sound much but…

Two-word and three-word may mean just that e.g. go bus and man go bus. But the words could also be considered two or three elements (or ideas). Each one of these can expand. So go bus might grow a bit for each element and become going red bus. And man go bus might grow to that man gonna go big red bus. Children expand each element and also learn extra new ways of making longer utterances.

See the symbol-reading examples for 2-words and 3-words.