The OnePlus X was OnePlus entrance to the budget phone market, at 5 inches instead of the One and 2's 5.5 inches.
The OnePlus 3T was unveiled on November 15, 2016 as a minor upgrade to the still relatively new OnePlus 3. The upgrade consisted the use of a newer SoC; the Qualcomm Snapdragon 820 was replaced with the Snapdragon 821.