The Australian pair finished well ahead of the pack at the end of the 82.2-kilometre stage, with Porte securing yet another victory at Willunga Hill.
Plapp, meanwhile, finished in front of stage one winner Luke Durbridge, who retained his grip on the overall standings, and was seen applauding Porte as the 35-year-old completed his ride.
"The rider of the day was Plappy in second," Porte said after the stage. "He could have easily come around me but he didn’t.
“It has just been a great race so far. I don’t have the form I normally would and I had to go longer than I normally would too, and that hurt, but it’s just great to be back racing.”
Porte's victory was the icing on the cake for Plapp after the 20-year-old took home stage 2 honours on Friday.
“It’s my first Tour Down Under," Plapp said. "I’ve always dreamt of doing it, I have been living in Adelaide for the past two years.
"Unfortunately I missed out last year. To be here this year, even though it is a world tour race, it’s still pretty awesome.”
Sarah Gigante also tasted victory in her respective race and now leads all four classifications ahead of tomorrow's final-stage circuit around Adelaide.
The 20-year-old was at her best yet again for the hill climb and finished a full minute ahead of WorldTour climber Lucy Kennedy (Team BikeExchange), who was second ahead of Nicole Frain (Sydney Uni Staminade).