How to create unique index on multiple columns where the combination doesn't matter?

Hello,

If I have a table like

CREATE TABLE relationship (
obj1 INTEGER NOT NULL REFERENCES object,
obj2 INTEGER NOT NULL REFERENCES object,
obj3 INTEGER NOT NULL REFERENCES object,
...
)

And I want to constrain that if 1,2,3 is already in the table, rows like 1,3,2 or 2,1,3 shouldn't be allowed.

Is there a general solution to this problem?

Sorry if the question is too basic, but I couldn't find the answer in the doc, at least not in the chapter on unique index.