As an opening Modifier
A 3♦ opening is preemptive in every system.
As a response Modifier
The 3-level responses of a 1NT opening have many possible meanings, which depend on agreements.
2♣-3♦ generally shows 8+ HCPs and a 6-card or good 5-card ♦ suit.
A single raise of a weak two bid is preemptive and to play.
This shows a strong hand and a self-sufficient suit, and is forcing.
As an overcall Modifier
A jump overcall of 3♦ is usually equivalent to a 3♦ opening, which is preemptive.