The "Premptive 3's" (or Weak 3's) are very aggressive, defensive opening bids of  3♣ ,  3 ♦ ,  3 ♥  and  3 ♠ . The opening 3-level bids are greatly disruptive because they leave less bidding space for the opponents to communicate and to find a suit to bid.

These hands hold 9-11 HCPs and hold exactly 7-cards in the named suit. Since an 8+card fit is needed to control a suit, the partnership is extremely close to finding a suit for a contract.

The Preemptive 3 opening commits the partnership to taking 9 tricks. The opener should have at least 6 tricks in the hand. The opener hopes the responder will make up the difference by taking at least 3 tricks.

There are 4 requirements for a Pre-emptive 3 (Weak 3) opening bid:

Point Range:
9-11 HCPs
A minimal hand with 8 or less HCPs is very risky. Strongly consider a PASS with fewer than 9 HCPs.

Suit Length:
Must have exactly 7-cards in the opened suit. No more, no less.

Suit Strength (Critical):
The opened 7-card suit must have at least 5 honor card points (HCPs) (2 of the 3 top honors). Many players forget the honors in the named suit, and end up losing the contract.

Suit Selection: All 4 suits can be opened with this bid.