Is it worth doing this 3 hour live test?

I recently accepted a full-time design job after a long search. At the same time, an agency I was interviewing with (project-based role) asked me to join an interview where I’ll complete a 3-hour live design assignment as part of their process. I asked that I have a full-time job now and if I could do it on my own time or if there’s flexibility, but they said it has to be live or they’ll move on.

They reached out to me first specifically because they liked my portfolio, so why do I need a test? And why can’t they at least give me a paid trial period? My main worry is spending time on this test then being ghosted, especially considering it’s a new startup and they only have one negative review on Glassdoor about being ghosted.

I had previous tests where it was paid but this one isn’t. The pay is good for this job and I wouldn’t mind some extra income with good clients but also not sure if it’s worth pursuing if I’m already starting a good full-time role.