Difference among 走去 vs 走路 vs 步行? Are 走去, 走路, and 步行 interchangeable? Specifically, are they all valid variations of saying "to walk?"?

For instance, are all these identical?


我們走去了三分鐘
我們走路了三分鐘
我們步行了三分鐘
我們走去了三分鐘 and 我們走路了三分鐘 are unnatural. The common way to say it is 我們走了三分鐘.  

走去 is not a good set. We don't really use it. 

More often, 走路 is more like a gerund, and 走 alone is usually used as a verb. You can say 我們走路走了三分鐘. 

步行 could be used as a verb as it's in your example.

So, these sentences convey the same thing:


  我們走了三分鐘.
  
  我們走路走了三分鐘.
  
  我們步行了三分鐘.