How to import data from a dump file to a specific schema in postgres?
probably the easiest is to first rename the existing schema_1

restore the dump

rename the new schema_1 to schema_3

restore the original schema_1 to its original name

eg psql:

 alter schema schema_1 rename to schema_1_backup;
 create schema schema_1;
 \i schema_1_dump
 drop schema_3 cascade;
 alter schema schema_1 rename to schema_3;
 alter schema schema_1_backup rename to schema_1;


This is going to annoy other users.

The other way is to change all the SQL commands to say  'schema3' instead of 'schema1'