The open three is the most important shape in gomoku. It is the last stage before an unstoppable open four, so it demands an answer, and the way you answer it shapes the next several moves. Blocking an open three is easy. Blocking it well, so that the attacker's follow-up comes to nothing, takes a little more thought.
Two kinds of open three
A solid open three is three stones side by side with empty points at both ends: for example X on g8, h8 and i8, with f8 and j8 empty and room beyond them.
A split (or broken) open three has a gap: X on g8, h8 and j8, with i8 empty in the middle and both outer ends open. It is just as dangerous. If X fills the gap, the result is an open four.
Both shapes need an answer now. The difference is where you can put that answer.
Blocking a solid three
A solid open three has two natural blocking points, one at each end. Either one turns the shape into a closed three, which is no longer an immediate threat. Choose between them by asking:
- Which end is closer to other X stones, on the same line or crossing lines? Block that end; it is the direction the attacker wants to grow.
- Which end touches your own stones? A block that also makes you a two or three is worth more.
There is also a third option some players forget: blocking one point further out. Placing O one point beyond the end (for example on e8 when the three is g8–i8 and f8 is empty) does not stop X from making four, but it makes that four closed. This is usually worse than blocking directly, because it lets X gain a tempo with the closed four. Prefer the direct block unless you have a clear reason.
Blocking a split three
For a split three there are three candidate points: the gap and the two outer ends. Blocking inside the gap is often the strongest choice, because it cuts the line into a two and a one that can never reconnect. Blocking an outer end leaves the attacker the gap, and filling it produces a closed four, which gains the attacker a tempo.
That said, the gap is not always best. If an outer end is where several of the attacker's other lines cross, blocking there may prevent a bigger problem. As always, look at the whole board, not just the line.
The follow-up attack
Blocking the three does not end the attacker's plan. Good players create an open three precisely because they know the block will go to one of two or three points, and they have prepared a second line that benefits from your forced move.
The typical pattern: the attacker's three shares a stone with another line. You block the three; the attacker plays on the other line and makes a new three, or a four. If your block did not touch that second line, you are defending again, and the attacker keeps the initiative.
So before you block, look at every stone in the three and ask which other lines pass through it. If one of your candidate blocking points also sits on one of those lines, choose it.
When the three is part of a four-three
Sometimes the open three appears at the same time as a closed four, created by a single stone. This is the four-three, the classic winning combination. You must block the four first (otherwise X makes five), and then X extends the three into an open four. At that point the game is usually lost.
The defence against a four-three happens a move earlier: spotting the point where one stone would create both shapes, and occupying it before the attacker does. See surviving a four-three for the details.
The blocked ends rule and open threes
Under the Vietnamese blocked ends rule, a five needs at least one open end to win. That does not change the basic answer to an open three, but it adds an option in some positions: if the three is already hemmed in at a distance by your stones, a four built from it might end up closed at both ends and therefore worthless. Experienced players count these distances before deciding whether a block is urgent. Beginners should simply block.
Reading the board before the three appears
The best defenders rarely face an open three by surprise. They see the open two that will become it, and they judge whether that two is worth a stone yet. An open two is not a threat by itself, but an open two that points at a crossing with another enemy line is a warning: one stone could produce a three and something else at once.
A simple habit helps. When an enemy two appears, look at the two points that would extend it into a three on each side. If either of those points also lies on another enemy line, that point deserves attention. You do not have to block it immediately, but keep it in mind, and if the opponent's next move adds to either line, take it.
Another useful habit is to count your own stones near the enemy two. If you already have stones close to one end, the two can only grow in the other direction, which makes it much less dangerous. Many "sudden" threes come from twos that had room at both ends because nobody bothered to put a stone nearby.
Practising the pattern
Open threes appear in almost every game, so practice is easy to find. Play the computer on the home page at the second or third level, and after each game count how many open threes each side made and how each was answered. You will quickly see which of your blocks kept the game quiet and which let the attacker continue. More defensive topics are collected in the defense section.
Frequently asked questions
What is the difference between an open three and a closed three?
An open three has room to grow at both ends and becomes an open four in one move. A closed three is blocked at one end and needs two moves to become dangerous.
Where should I block a split three?
Often inside the gap, which cuts the line permanently. An outer end can be better if it also stops the attacker's other lines.
Can I block one point away from the end?
You can, but it usually lets the attacker make a closed four and gain a tempo. Blocking directly is normally better.
Why does the attacker keep getting new threes after I block?
Because their three was connected to other lines. Before blocking, check which lines pass through each stone of the three and choose a block that touches them.
Is an open three always a must-answer?
Yes, unless you can play a four first. A four forces an answer; your own three does not.
