Salesforce Object Query Language (SOQL) does not support JOIN operations that SQL does. The reason for this is the multi-tenant nature of the Force.com platform. https://developer.salesforce.com/page/Multi_Tenant_Architecture
SOQL does have a feature called Relationship Queries. https://www.salesforce.com/us/developer/docs/dbcom_soql_sosl/Content/sforce_api_calls_soql_relationships.htm
You can use Parent / Child Queries to save yourself SOQL calls and improve performance, however you cannot go more than 1 level down to Children and more than 5 levels up for Parents.
So you can do for example:
Select Id, Name,
(Select Id, Name, Object_1__r.Name from Junctions_1__r),
(Select Id, Name, Object_2__r.Name from Junctions_2__r),
(Select Id, Name, Object_3__r.Name from Junctions_3__r)
from Main_Object__c
Which is letting you get the data in one query.
You will still need to use MAP's in APEX to JOIN data across the junctions.
I hope that helps you.
