think of it just like joining two tables together. It is really the same thing. You are basically joining the result set of the first join with the 3rd table.
select * from tablea a inner join tableb b (on a.id = b.id) inner join tablec c on (b.id2 = c.id2);
I believe this would work the same:
select * from tablea a, tableb b, tablec c where a.id = b.id and b.id2 = c.id2
There is also natural inner joins will give you a 'cleaner' output if the names are the same in the tables
to do anything further you will have to give some more info This will only return results the match the 'on' restrictions in each join.
Here is a good ref: http://en.wikipedia.org/wiki/Join_(SQL)
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