Can I adjust the Peek and Pop transitions of 3D Touch?

Yes. You can adjust the amount of pressure for the Peek and Pop transitions of 3D touch by navigating to Settings > General > Accessibility > 3D Touch. "Peek and Pop" are what Apple calls the features of 3D Touch.

Peek allows users to preview items like email without opening it fully. It also allows users to swipe up to open Peek quick actions, which provide a dropdown box of functions such as Reply, Forward and so on.

Pop is the same as clicking a link to open an email or opening links to websites in Safari.