Then they give you your real chair.
It is probably realistic to expect someone to write something useful (although possibly small) in three days. It is less realistic to expect someone to write a useful component integrated in a large system that they have to learn, in three days.
Is that actually true? It's probably uncommon in tech but I understand it's fairly common in creative fields and--while it was a long time ago--I worked a couple unpaid internships when I was in high school in chemistry labs.
A railroad engineer intern can move cars around the yard all day, assembly trains, and other such things. However if they move the car to a loading dock where it is loaded/unloaded that is valuable work and must be paid. If the train they assembly leaves the yard for the next that is valuable work (unless they all make it back to this yard unused).
Most companies choose to pay interns rather than figure out how to not break the rules. A few have a separate intern program where they carefully figure out tasks to assign interns for training. If your intern program did valuable work you should talk to a lawyer.
See also: college admissions. (No, that kid probably didn't do that volunteering activity because they wanted to help people, but because they were told it was a checklist item, as they were coached through college admissions. And they probably didn't learn anything, and they were coached on what to say about that in the essay. And, normally, actually productive volunteering probably wouldn't have looked like each one of those kids starting their own duplicate initiative so each could spin it as demonstrating leadership. And it's more an option for kids from families well-off enough that the kids didn't have to work jobs that would contribute better to their household income or college expenses. And don't get me started on how the well-to-do decided that "travel" would be a plus on applications.)