On Steam there are some users telling me that when they start the game with 3 monitors, the game is spreading on all of them.

But when the game starts I'm using this:
video_set(sys_metrics(0), sys_metrics(1), 32, 1);

So the game should run only on one monitor, but the problem occurs when users are using Nvidia's "Surround" mode, then this function thinks that all 3 monitors are one.

So, is there another way to detect the correct resolution of the main monitor?

In that mode sys_metrics() probably includes the area of all monitors. You need DirectX for determining the monitor size. I have not yet had this case, but try GetAdapterMonitor() and GetAdapterDisplayMode().