Thank you all,
If so, what are the addvantages in triangle configuration that still manufacturing such FLT's?

Trade Show Dave is absolutely correct, but here is a less technical answer:

Another way one can look at it, is this:

There are 3 support points, or suspension points, that support the forklift onto the frame of the truck before the truck hits the ground. One is located at each hub of the front drive wheels, and the third, at the pivot box in the center of the rear axle. The 2 rear steer wheels are not connected to the frame of the truck with shocks, struts, etc, There is not a direct connection of the rear wheels onto the frame. But the pivot box is mounted onto the frame, hence the third support point.

Play connect the dots amongst the three support points, voila, you've got yourself a triangle, known as the stability triangle.

Basically....
The reason a forklift has three points of stability instead of four is: the steering axle on a forklift is connected in the center only, to the rest of the forklift.
This is so that the lift truck can drive over a small dip, or driveway and still have all four wheels on the ground.
This also makes turning on a ramp or hill very dangerous because the center of gravity needs only move a few inches to be outside of the "stability triangle" to cause the forklift to turn over.
An unloaded truck is less stable than a partially loaded truck in relation to the location of the center of gravity and the increased possibility of side tipping.