How to wire a 3-way dimmer switch in box with three cables
What you have there are cables, not conduits. 

A 3-way switch has 2 travelers that must necessaily be in the same conduit or cable.   On your setup, that can only describe the white and red from the upper cable/conduit.  

3-way switches have 1 more terminal (other than ground), which is called common. Lo and behold, there is one loose wire left in your panel.  

Make sure you are using the right size of wire nut.  The right one for these connections is orange or yellow.  Red is too big.  

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Travelers are very confusing.  Maddening, in fact, because they are typically wired using common cables, and those come in stock colors (black white and red).  As such, in standard cables, color does not correspond to purpose. This means you have (for instance) white wires in the box that are neutral, and other white wires in the box which are travelers.  In a comment, you ask a very simple question, expecting a simple answer and there isn't one.  So we're gonna make it simple.  

Those two wires in the same conduit/cable are both travelers.  We are going to get yellow tape and re-mark them with yellow tape.  This is my standard color for travelers, since it matches the brass screw on common 3-way switches. 

Since the two red wires on the switch also are travelers, we remark them also with yellow tape. 

When I run traveler wires in actual conduit, where I get choice of wire color, I use 2 yellows for travelers. There is no need to distinguish them from each other.