With our second baby due soon, should we continue trying to potty train our three-year-old daughter? I wonder whether she’ll simply regress when her brother comes along.

Yes, you can continue to potty train. She might have regression like what you have mentioned but not very great impact. My daugther saw her brother wearing newborn diaper she also want to follow. She just curious and want to have fun too. But she still continue to use potty at the same time. It's good to start before the arrival of newborn as you will get very busy once baby is here.