You can rephrase your question to: "The work I've been given to do is not challenging". That is very different. If it isn't challenging then do you have spare time, or are you bogged down with huge piles of meaninglessness? If it's the former, start making yourself invaluable. I certainly wouldn't give a new hire something critical to do until I know how well they are doing and what their skills are, so don't expect to be CEO in three days. However, there are probably projects going on that you can start learning about and making an impact in.
If it's just piles of work that really are boring, two things: first, you chose the job, so evaluate that. You said that the previous job wasn't challenging either. Maybe it's you? What are your standards for a "challenging" job? Why didn't you find one? Second, boring work is screaming for efficiencies. As before, make yourself invaluable by focusing on streamlining processes. Can you automate parts of it to free up time? Can you improve on what is done and when? Can you allocate resources more efficiently?
Three days is not long enough to decide if you like the job and should stay, but it is long enough to start thinking about how to take ownership and excel at the position.