Three in a row is where gomoku gets serious. A line of two is a promise; a line of three is a threat — or at least, some threes are. The most important distinction for any gomoku player is between an open three, which must almost always be answered at once, and a closed three, which usually can wait. Mixing them up is the fastest way to lose games or to waste moves defending against nothing.
The open three
An open three is three of your marks on a line, with enough empty space on both sides that one more mark would make an open four — four in a row with both ends empty.
Why is that so important? Because an open four cannot be stopped. It threatens five at two different points, and the defender can only block one of them.
For a solid open three like g8–h8–i8, you need empty points at f8 and j8, and at least one more empty point further out on one side (e8 or k8). If both f8 and j8 are empty but e8 and k8 are both blocked, extending gives a four that is blocked at one end — not an open four. In practice, on an open board the extra space is almost always there.
The closed three
A closed three (sometimes called a "dead" or "sleeping" three) is three in a row that is already blocked at one end — by an opponent mark or by the edge. Adding a fourth mark can only make a closed four, which threatens five at a single point and can be blocked with one move.
A closed three is not harmless. It can become a closed four, which forces a reply, and forcing replies are the raw material of combinations. But on its own, a closed three does not need to be answered immediately.
The broken three
There is a third kind of three that looks different but behaves like an open three: the broken three, such as X X _ X, with open ends. Filling the gap makes an open four. Many beginners overlook broken threes because the gap breaks the visual line. They are covered in detail in broken threes.
Which to answer, and when
The basic rule is simple:
- Open three (solid or broken): answer it now, unless you can make a four of your own first. After your four is answered, you still need to deal with the three.
- Closed three: usually ignore it for now, unless it also sits on another dangerous line. Use the move to build or to block something more urgent.
Beginners tend to make the opposite mistakes: they panic at closed threes, wasting moves, and they overlook broken open threes. Learning to classify threes correctly frees up many moves for your own plans.
Where to block an open three
An open three has two ends, and sometimes a third option (for broken threes, the gap). Choosing well matters:
1. Block the end that also does something else. If one end sits on another of the opponent's lines, blocking there cuts two lines at once.
2. Block the end that helps your own shape. If one end sits next to your existing marks, the block also builds your line.
3. Watch what the opponent's next forced move would build. After you block one end, the attacker may push to a closed four at the other end. Where would that four-mark land? If it would create a second attacker line, block the other end instead so that the continuation is harmless.
4. For broken threes, the gap is often best. Playing in the gap splits the shape completely, leaving two separate pieces instead of a three with one end blocked.
How the rule set changes things
In freestyle, a closed three can still grow into a closed four and eventually a five as long as there is room.
Under blocked ends (Vietnamese caro), closed threes are even weaker. A closed three that grows into a five will have the opponent's mark at one end — so if the defender later places a mark at the other end, the whole line is dead forever. Blocked-ends defenders often use this: they answer an open three by blocking one end, knowing that one more mark later will kill the line completely. See the blocked-ends rule.
Threes in combination
Threes become truly dangerous when they combine with other threats. A single move that creates an open three and a four (a four-three) or two open threes (a double three) usually wins, because the defender cannot answer both. When you scan the board, pay special attention to any point where the opponent could create a three while also making another threat. Those points often need to be occupied before the combination appears. The attack side of this is explained in four-three combination and double-three attack.
A classification exercise
Take a pencil and squared paper and draw these lines, then decide which are open threes:
- _ X X X _ with empty space beyond both ends.
- O X X X _ _
- _ X X _ X _
- _ X _ X _ X _
- O X X _ X _
Answers: 1 is an open three. 2 is a closed three (blocked on the left). 3 is a broken open three — filling the gap makes an open four. 4 is not a three in the dangerous sense: two gaps mean no single move makes an open four. 5 is a closed broken three — filling the gap makes a closed four.
Doing a few of these a day for a week makes classification automatic.
Practice in real games
Against the computer on the home board, play a few games where your only focus is classification: every time a three appears, decide within two seconds whether it is open or closed. If open, block it well; if closed, use your move for something else. Replaying your games from your account will show you how many moves you saved.
Frequently asked questions
What is an open three in gomoku?
Three marks on a line with enough empty space on both sides that one more mark would make an open four.
Do I always have to block an open three?
Almost always. The exception is when you can make a four of your own first; even then, you must usually block the three afterwards.
Is a closed three dangerous?
Not on its own. It becomes dangerous when it combines with another threat, because it can turn into a forcing four.
Which end should I block?
The end that also cuts another enemy line or builds your own, while making sure the attacker's next forced move does not build a second threat.
Is a broken three an open three?
Yes, if the ends are open. Filling the gap makes an open four.
