It will be three years by coming Januaryでも伝わります。こう言う前に「We have been dating for a while＝しばらく付き合ってるよ」などと付け足すとばっちりです。
It'll be our third year anniversary next January＝来年の１月で３年記念日です
We've been dating for about three years＝３年くらい付き合ってるよ
詳細なしでストレートな伝え方です。彼女が横で聞いてる場合は前者の方がお勧めです（笑）。
This January we're celebrating our 3rd anniversary.
Native speakers will say like this: “We will be celebrating our third anniversary this January”.
There are lots of other ways to say it, but this one is the most accurate and most commonly used.
Here is an example on how to use this in a conversation:
A: How long have you two been together?
B: We've been dating for a long while! It will be our third anniversary in January.
ネイティブスピーカーは次のように言うでしょう：
“We will be celebrating our third anniversary this January”
（1月で3年になります）
他にもいろいろな言い方ができますが、これが最も正確で一般的です。
以下、会話例です：
A: How long have you two been together?（二人は付き合ってどのくらいになるの）
B: We've been dating for a long while! It will be our third anniversary in January.（付き合い始めてずいぶんたちます。1月で3年です）
Explanations of how long you have been in a relationship can be made by saying:
It will be our third anniversary
We will have been together for three years
It's our third anniversary in January
どのくらい交際しているのか伝える言い方です：
It will be our third anniversary
付き合って3年になります。
We will have been together for three years
付き合って3年になります。
It's our third anniversary in January
1月で3年になります。
We will reach our three year anniversary next year January.
We will celebrate three years anniversary next year January.
Any of these sentences can be used for this:-
1. We will reach our three year anniversary next year January.
2. We will celebrate three years anniversary next year January.
Both these statements mean the same thing, and that is, nexy year January, your relationship would have existed for three years.
これらどの文も使うことができます：
1. We will reach our three year anniversary next year January.
来年の1月で3年になります。
2. We will celebrate three years anniversary next year January.
来年の1月で3年になります。
どちらの文も同じ意味です。来年の1月で交際して3年になると言っています。
As a native English speaker I would express this by saying "It's our three year anniversary next January".
Ex: How long have you been together?
It's our three year anniversary next January.
英語のネイティブとして、私なら次のように言います：
"It's our three year anniversary next January".（1月で3年になります）
例：
How long have you been together?（付き合ってどのくらいになるの？）
It's our three year anniversary next January.（1月で3年になります）
You don't always need to specify the month. You can use expressions like "Our 3rd anniversary is just around the corner." Which means after a short time (the corner) it will be your anniversary.
You can also say "we're coming up on our 3rd anniversary." Which means the same thing as "around the corner" but using the words "coming up on". "Coming up on" is similar to "going towards" or "approaching".
If you want to mention January. You can say "It'll be 3 years in January". To make it more sentimental: "I can't believe it'll be 3 years in January,"
必ずしも「月」を明確にする必要はありません。例えば、"Our 3rd anniversary is just around the corner"（もうすぐ3周年です）のように言えます。もうすぐ3周年だという意味です。
"We're coming up on our 3rd anniversary"（もうすぐ3周年です）と言うこともできます。これは "around the corner" と同じ意味ですが、"coming up on" を使っています。"coming up on" は "going towards"（～に向かっている）や "approaching"（～に近づいている）に近い意味です。
1月（January）に言及したいなら、"It'll be 3 years in January"（1月で3年になります）と言えます。より感傷的に、"I can't believe it'll be 3 years in January"（1月で3年になるだなんて信じられない）と言うこともできます。
アメリカ合衆国 
カナダ 
イギリス 
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