Multiple Foreign Keys

I was wondering if there is any way to set up a foreign key that is
valid not just if the value exists in one table but if it exists in any
of 3 tables. Here is what I mean. I have a sequence called 'id_number'
that I start at 1 and increment by 1 every time its used. I then have 3
tables each of which has an id attribute the is set up like this:

id int PRIMARY KEY default nextval('id_number'),

My question is this. I have a fourth table that also has an id field
but I want to constrain it so that it must be a valid id from either
table 1, or table 2, or table 3. I cannot seem to set it up except for
making it be in all 3 which obviously could never happen. I'd
appreciate any advice you could provide me in this regard. Maybe it's
not possible, but if there is a way it would sure save a lot of hassle
in otherwise having to redesign the DB.

Thanks

jason j burke