Take a 3 minute file, let say it is 340 MB in size, divide that with the 180 seconds that make up 3 minutes, and you have the amount of Megabytes / second the camera have been writing at.
Memory cards as you know list their speeds in Megabytes/ second.
In this example that would be a write speed of 1.88 MB/ second the camera have been writing.
Off course if the camera use variable bitrate you will see different file sizes, but daytime driving should provide the worst case scenario, night time recordings file size can be significant smaller with a VBR camera.
If it is a VBR camera then in parts of that 3 minute recording the camera might have been writing at 3 MB/s and then other parts it might only have been writing at 0.8 MB/s
The above are the average write speed you are getting, but if your action in the footage have been the same thruout the 3 minute recording you can also be sure the bitrate the camera have chosen to use have been pretty much the same.
Each 3 minute file is around 360Mb,
There are 8 bits in a byte, so 360 x 8 = 2880 Megabits of information to write to the card over the 3 minute period,
There are 60 x 3 = 180 seconds in 3 minutes,
This means that 2880 Megabits need to be written over 180 seconds leaving an absolute minimum write speed of 16Mb/s.