Users wraps message A in three layers of encryption, result is message D.
Node 1 decrypts message D into message C.
Node 2 decrypts message C into message B.
Node 3 decrypts message B into message A.
Server receives message A from Node 3.
Question: isn't it possible, having message A, to ask Node 3 what the message received was and who sent that message? Now we know about message B and Node 2, repeat - now we know [message C, Node 2], repeat - [message D, Node 1], repeat - User's IP address!
Tinfoil hat time. There are around 8000 nodes which is not that much, there are not a lot of countries hostile to USA, intelligence agencies share information with each other, independent nodes can be shut down or compromised.