SQL & PL/SQL :: Update With Join Over Two Tables

I am trying to write an Update that really frustrates me because it won't work for one reason or another.The situation is that I have two tables for customer information, t1 with the names of the customer and t2 with the address.These two can be joined via a client_id.

Now I have a third table t3 with the name and address of potential customers. I want to find out if some of them are already known to me so that I can update the client_id from table t1 or t2 into t3.

I have to join firstname, lastname from t3 to firstname, lastname from t1 and street, zip, city from t3 to street, zip, city from t2 and client_id from t1 to t2. Additional there is the problem that there can be more than one result so I have to update one of the found client_ids per name/address into t3.I am no expert to PL/SQL, I just know what SQL works in Access and that is:

UPDATE (t3 INNER JOIN t1
ON (t3.firstname= t1.firstname) AND (t3.lastname = t1.lastname)) INNER JOIN t2
ON (t3.city = t2.city) AND (t3.zip = t2.zip) AND (t3.street = t2.street) AND (t1.client_id = t2.client_id)
SET t3.client_id = t1.client_id;