The Weak Three Opening Bid (3C or 3D or 3H or 3S) is a preemptive bid. It shows a 7 card suit, 2 honours in that suit, no other 4-card Major and a weak hand (6-10 HCP).
Weak Threes have the advantage of preempting the opposition and making it difficult for them to find a contract.
Good Suit Quality. When using this bid, have at least two honours in your long suit (count the 10 as an honour if you have the Ace).
Don't make this bid if you have another four card major suit. Partner may also have 4 cards in that major and you are likely to miss the fit.
First check if you could have open at the One level with the Rule of Twenty.
The Opener at 3 level must not bid again unless asked by the Responder. You have described your hand.
Don‘t make this bid in 4th seat after 3 passes unless you have 10-13HCP. Remember you need 19-24 HCP to make part game.
A preempt at favourable vulnerability shows about 5 tricks. You can afford to overbid by 4 tricks.
A preempt at equal vulnerability shows about 6 tricks. You can afford to overbid by 3 tricks.
A preempt at unfavourable vulnerability shows about 7 tricks. You can afford to overbid by 2 tricks.
If partner opens 3C or 3D, think about possibility of 3NT, if you have 2/3 of their suit (for communication) and stoppers in the other suits.
Either put up or shut up! Without support it is usually best to PASS. With 3 card support, consider raising to 4H/4S. However, if you have a good fit and no defensive qualities consider bidding to the level of the fit.
A change of suit response at the 3 level is forcing.
Go for an Attacking rather than a Passive defence as the suits are likely to split badly.
Make a Take Out Double over a preemptive Opening Bid if you would have done so over a One level Opening Bid.
Make a 3NT overcall over a preemptive Opening Bid if you would have bid 1NT over a One level Opening Bid.
If you can‘t overcall/takeout/bid a new suit, don’t be afraid to PASS even with a strong hand.
With an 8th card in your suit, open at the four level.