There are apparently bits of possible foreshadowing for this, based on what Stephen Broussard said during his appearance on The Empire Film Podcast, and there’s an implication that the Quantum Realm contains more waiting to be discovered, especially where Janet van Dyne is concerned.
So, while there is currently no guarantee that Ant-Man 3 (or Ant-Man and The Wasp 2) will actually happen, there seems to be a basic roadmap in place if Stephen Broussard, Peyton Reed, and the film’s team decide to head back in for another.
For now, we do know that the continued exploration of the Quantum Realm could play a role in future installments, with a more in-depth examination of Janet’s time trapped there helping to inform the direction of the story.