We did the 3-day potty training with DD when she was almost 3 years old and DS was 5 months. Both DH and I were involved in the potty training so it was fine, but I can see how it would be difficult for one person to try the 3-day method and care for a newborn. With the 3-day method you want to try and catch every single accident as it is happening. That could be very hard to do if you are also trying to care for an infant.
We just did the 3DPT a few weeks ago, and I was by her side practically all day, each day. It would have been near impossible to care for an infant and give DD all the attention & constant reminders. So, yes, I agree. I'd arrange for care for your youngest if you're going to do the 3 day method.
Re: 3dpt Q & A