Интересная конструкция, то что он не двухосный, а трёхосный.
Так было более надёжно или в чём смысл столь нестандартного устройства вагона?

Sorry for the answer in English.
2 interesting books existing about this subject, written by Henry Elsner (volume 1 and volume 2): https://www.robertgavora.com/pages/books...ory-and
These books give an interesting insight into this technology.

Now to Belgium. 2 companies have applied this technology: the N.M.V.B. (interurban trams) and the T.E.G./M.I.V.G. (city trams in Ghent).

Two-axle trams were the direct successors of coaches and horse-drawn trams, and both comfort and stability over crossings and in curves were mediocre.
Very quickly, it was discovered (first in America) that 4-axle vehicles were the solution, but unfortunately this could not be applied to smaller trams with a short chassis.
So a possible solution was to build an additional axle in a two-axle vehicle.

The N.M.V.B. therefore experimented with this, but very quickly came to the conclusion that it did not offer a solution to their typical problems. They were long, straight routes, where often quite high speeds could be reached, but which was therefore not very comfortable with two-axle vehicles. In the stations and depots, the switches were of poor quality and therefore regularly resulted in derailments. The conversion to 3-axle vehicles did not solve the problem.
In the end, it was decided to put a standardised 4-axle tram into service and, where possible, to convert existing trams to 4-axle.
https://transphoto.org/vehicle/542041/#n713248
This tram also became a four-axle unit, but remained an oddity: it never became a 'standard' because it was shorter than the real four-axle units!

Now Ghent: unlike the N.MV.B., this was and still is a real city tram, with many curves and very low speeds. Here, the third axle did solve the problems of instability in the curves and the switches, and at slightly higher speed (quod non) the tram was also more stable. They were also trams of the very first design (from 1905) and that modernisation (+/- 1930) was considered very successful there.

It is also interesting to know that actually only trams of the first generation were built as double-axles, but that afterwards new 3-axles were built, e.g. in Munich.
It has certainly not been successful everywhere (see also N.M.V.B.), different techniques have existed, but: the aforementioned book provides a lot of information on this.