Association &3 is used redundant, it is used in &1 and &2.

What causes this issue?

The usage of associations in CDS path expressions is equivalent to
technical join operations in the native database view. If the same
association is in views which are stacked on top of each other,
duplicate joins are created which are potentially unnecessary. In order
to improve the performance of CDS Views duplicate joins should be
avoided.

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Instead of using the same association multiple times in stacked CDS
Views it is more efficient to include all the columns required into the
CDS View at the bottom of the stack.
This warning can be suppressed by putting the pseudo comment
/*+[hideWarning] { "IDS" : [ "DOUBLE_JOIN" ] }*/
in front of one of the path expressions using the association at hand or
at the view itself.

Error message extract from SAP system. Copyright SAP SE.