It doesn't seem there is a "3 stream / 600Mbps Wireless-N" combination according to this.
The 1900Mbps spec could be marketing smoke&mirrors for this theoretical & concurrent throughput combination:
5Ghz Wireless-AC (1300Mbps / 3 stream) + 5Ghz Wireless-N (300Mbps / 2 stream) + 2.4Ghz...