As an opening Edit
A 3♦ opening is preemptive in every system.
As a response Edit
To a 1♣ opening Edit
1♣ - 3♦ is a double jump shift, which is either a weak jump shift, showing less than 6 HCPs and 7 ♦s, or a splinter showing support and void or singleton in ♦s.
To a 1♦ opening Edit
1♦-3♦ is a double raise, usually a limit raise. However, if inverted minor raise is in force, it shows 6-11 HCPs.
1♥/1♠-3♦ is a jump shift, which may be played as weak, showing less than 6 HCPs and 7 ♦s, or strong, showing a ♦ single-suiter with slam interest.
The 3-level responses of a 1NT opening have many possible meanings, which depend on agreements.
To a strong 2♣ opening Edit
2♣-3♦ generally shows 8+ HCPs and a 6-card or good 5-card ♦ suit.
To a weak 2♦ opening Edit
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 Edit
A jump overcall of 3♦ is usually equivalent to a 3♦ opening, which is preemptive.