A. H. Armstrong writes in the introduction to his translation of the Enneads of Plotinus the following (Plotinus George Allen & Unwin Ltd, 1953, p. 27):
We may sum up the general philosophical and religious situation in the age of Plotinus in the words of G. Quispel, [Gnosis als Welt-Religion, ch. 3, p. 26] Late antiquity appears to our mind's eye as a land of three rivers, traversed by canals and with bridges which make traffic possible ; but all the same three great streams appear distinctly, Gnosis, Neo-Platonism, and Christianity. There are innumerable interconnexions, but the three streams remain distinct, springing from different sources and flowing in different directions. And even when Christianity, after drawing into its stream a great deal of water from the other two rivers, flows on by itself, the result is not a mere syncretism or fusion. Christianity assimilates what it takes from the other two but remains itself.
Are there any critical references that would clarify the differences between these "three great streams"?
Perhaps these streams are a misunderstanding and the situation is not the way Armstrong describes it. Showing that would also be an answer.