Display nodes that reference the same node than current page
It can be tricky to orient yourself around this, but it is a fairly common request to list sibling nodes according to whatever criteria (usually Taxonomy and Entity references). 

The solution involving Entity references is to use two Relationships, one pointing to the common node (C), then another to point to the nodes that reference it (A) and then a Contextual filter that feeds the Views context on what node you're currently on (B). 


Create Views listing your content A
Add a Relationship Content referenced from field_ref_C
Add another Relationship Content using field_ref_C, and set it to use the Relationship from the previous step field_ref_C: Content
Add a Contextual filter ID (from Content Category) and set it to use the second Relationship (field_ref_C the one from the previous step) with these options:
When the filter value is NOT available:
Provide default value
Content ID from URL
When the filter value IS available or a default is provided
Specify validation criteria (optional)
Content
[your appropriate types - B]
Add another Contextual filter ID  but this time Do not use a Relationship:
When the filter value is NOT available:
Provide default value
Content ID from URL
More: Exclude  

This will exclude the node you're currently viewing from the list.  

If you want to check the output in Views Preview at this point enter the same node ID both times, for example to see what the block would output on node 14 enter 14/14.  

The first argument is taking the current node of type B and navigating through the connections, to the country C and back to content A you want to show, the second argument is excluding the current node from the list. 
Place your block in a region shown only on content B, don't forget to set the option Node from URL on block settings.