What is the QuickBase equivalent to a SQL join statement?

Quickbase uses "relationships" for joins between tables. Go to a tables settings and click relationships. You can change the reference field later or change the key fields.

I have a relationship with three tables. I want a report with every record from the two sub tables joined on every record from the parent table where the record ID is equal. This will display a lot of rows. Where and how do I create the report? 

If you have 3 tables you must have at least 2 relationships.
Just by default from those relationships you can create a report on the parent table and add the related child field which will give you a link/count of related records for that table, then same thing for the 3rd table.

Or you can create a reverse relationship to bring up child record values to the parent level for all the fields you want, and then add those.

If you're ok with custom script, that would be the easiest way since you can just query without creating relationships. If you want custom script help you can contact me directly. chuck@chuck.support