You need to concatenate table2 and table3 first and then right join it with table1.
load client, category from table 2
load client, category from table 3
from table 1
category from resident test
drop table test
The way you do it now won't work since after joining table1 and table2 the resulting table will have two fields and the join on table3 will use both fields for the joining because table3 also has these two fields.