is it possible to triple stack the shields
If the shields can physically stack without interference between adjacent shields, and as long as the total current on each pin is not exceeded, you can stack as many as you like. Each time you add a shield you have to consider conflicting pin assignments with the other pins already stacked, so whether the stacked shields would actually work when stacked would depend on the specific shields in question.
If you indulge in this, note that the most likely mechanical problem is fouling with an RJ45 socket on the board below. Note in your picture the motor shield is bare over the RJ45. This can also be fixed by having longer pins.
Picture shows a store-bought ethernet shield on a Mega, with two home-brewed shields above.