I think it's a no-brainer.
They have no reason not to continue, it's a golden mine and they know it, it simply takes a little while to animate a whole season.
Animation companies are always silent regarding whether they do or not the second season of a serie in case it's canceled for whatever reason (competition is harsh in Japan). But you shouldn't worry for Jojo, it's a huge success based on one of the main pillar of the whole Manga genre

Does anyone know if there are any plans to animate Part 4?