If you are going to buy Surface Pro 3 as portable workstation replacement, then you might care about the multiple monitor support in Surface Pro 3. Similar to previous Surface Pro devices, you can use Surface Pro 3 to drive multiple monitors simultaneously. But the resolution depends of the Surface Pro 3 SKU you are going to buy. Read about the Surface Pro 3 SKUs here. Only Core i5 and Core i7 models can drive displays up to 2880×1800 resolution each.
For those concerned about multiple monitor support, Microsoft engineers tell me they’ve tested these scenarios thoroughly. The i3-based model can comfortably drive two Full HD (1920×1200 at 60 Hz) displays, while the i5 and i7 models can power two external displays at a maximum resolution of 2880×1800 each at 60 Hz.
In all cases, you need to make the connection to the external monitors using the mini-DisplayPort adapter on the side of the device. For a single monitor, you can use an adapter to output to HDMI, DVI, or VGA. For dual displays you need either a DisplayPort hub or monitors that support DisplayPort chaining. In any configuration, you can continue to use the built-in touchscreen in addition to the external displays.