A 3" puffer in a 10g QT will be a disaster for water quality if you don't have a heavily seeded filtration. You will most certainly kill the puffer that way. My suggestion is to wait on the treatment until you get the QT fully cycled, if your fish are not in immediate danger. You can start now with a table shrimp in the 10g tank and some a little bit of sand from your DT to kick start. Once the cycle is over, put the sand back into the DT.
Also, I suggest that you maintain 0.5ppm of cupramine for 4 weeks straight to guarantee a high ich kill rate. I have tried different time frames, from two weeks to five weeks, and noticed that > 4 weeks always did the job, while 2 or 3 weeks sometimes don't.