三级门架也称为三层桅杆，是一种伸缩式桅杆，由一个外部固定框架和两个内部可移动框架组成。抬起叉车时，它首先抬升固定的外部桅杆通道的整个范围，称为“自由抬升”尺寸。之后，两个内部框架开始延伸-内部最领先。
该构造允许最小的塌陷或降低的高度以及最大可获得的货叉高程。当车辆必须在较低的车门开口或障碍物下进行协商或行驶时，必须能够达到较高的高度，才能发现这些优点。
A Three-Stage Mast also known as a triplex mast, is a telescoping mast comprised of an outer stationary frame, and two inner moveable frames. When lifting the fork carriage it first raises the full extent of the stationary outer mast channel known as the "free lift" dimension. After which both inner frames begin to extend - the inner most leading the way.
The construction allows the minimum collapsed or lowered height with the maximum obtainable fork elevation. The advantages are found when extended heights must be able to reach tall racks while the vehicle must negotiate or travel under lower door openings or obstructions.