I'm connecting 3 ASUS TUF Gaming 27" monitors to a dell 7490 latitude using WD19TB docking station but only 2 monitors are working.

The GPU in that system can drive only three displays -- it may work, but you'll need to disable the internal display. And given the age of the CPU/GPU coupled with the resolution of the displays, this system isn't going to be ideal for running three displays.  

Does it mean I wont be able to use it with 3 monitors? Is there any docking station compatible with Dell Latitude 7490 to support additional 3 monitors?

If you mean three monitors in addition to the notebook display, no -- that's not a function of the docking station.  It's a limit of the system itself.

And if that docking station is connected to a USB 3 port, and all three are high resolution monitors, you'll run into bandwidth issues with the setup.