If R7 M370 is the only one GPU in system i think it can use two or three displays in common case:
Laptop panel + one of external outputs or both external outputs. But if your Laptop panel has inner DP connection, then probably you can use three monitors(Laptop panel + both external outputs)
In your case laptop panel is connected to Intel graphics.Probably one of other outputs (VGA or HDMI) too. You really should contact DELL for explanations of your laptop video output system.
It is limited not by AMD hardware capabilities but by design created by DELL engineers.