Improving knowledge of CC rules/movement during fight phase

[Morticon]

Is there a specific rule that reifies the three points of contact to "trap" enemy units in combat? Anyone know where I could find it?

 

I got into a bit of a squabble with a Tau player over this, but couldn't find it in any rules directly stated.  I had some of his fire warriors trapped, but then lost the "roll off"

 

 

You got cheated if you had to roll off for this. 

 

As mentioned above, its a movement rules.  You cant move through an enemy as per movement rules. If you have three points locked on his model, where can he move?