Space

From what I recall (I may also be wrong) they used a (heavy spoilers for three body problem trilogy) >!photoid to destroy only one of the three stars that Trisolaris was orbiting because there was no where the Trisolarans could hide considering that was the only planet in their system. A dark forest strike using a dual vector foil was used against the solar system due to the presence of "blind spots" which I took as a reference to our nine planet system as opposed to the one planet system of Trisolaris. We had giant, distant gas giants we could hide behind to shield ourselves from a photoid strike whereas Trisolaris was a solitary planet with no where to hide.!<

But yeah if >!the three stars were far enough away to have their own separate planetary systems then you would absolutely need three separate dark forest strikes to ensure you got rid of all potential civilizations. But what I got from the Three Body Problem trilogy is that a dark forest strike is meant to be as efficient and low cost as possible, if one photoid could destroy all life in the system then that's what they would do, but if there was a concern that they could potentially survive a simple photoid strike then they would use something more advanced like a dual vector foil to ensure there were no survivors that could try and take revenge. If the systems were far enough apart then they would absolutely strike all the systems separately as opposed to half assing it, but the Singer chapter made it pretty clear to me that efficiency and low effort was key to a dark forest strike.!<

That's just my opinion and what I got out of the books though :)

Edit to add - I think we both agree with each other, I'm just happy to run into another fan of TBP trilogy, some of the greatest books out there besides The Expanse ;)