I don't think the weather forecast is accurate when it comes to predicting the weather from the 3rd day onward.
ご質問ありがとうございます。
「3日後以降の[天気予報](https://eikaiwa.dmm.com/uknow/questions/26180/)はあまり当たらないと思うよ」は英語で「I don't think the weather forecast is accurate when it comes to predicting the weather from the 3rd day onward.」と言います。
また、「天気予報は明日・明後日ぐらいまではだいたい当たると思う」と説明したいなら、そうすると、「I think the weather forecast is only accurate with predicting the weather for tomorrow and the next day.」になります。
ご参考になれば幸いです。
The weather forecast isn't usually accurate for more than a few days.
It's hard for weather forecasters to predict the weather for more than three days in advance.
この場合、次のような言い方ができます。
ーThe weather forecast isn't usually accurate for more than a few days.
「天気予報は数日以降だと大抵当たらない」
isn't accurate で「当たらない」と言えます。
ーIt's hard for weather forecasters to predict the weather for more than three days in advance.
「天気予報士が3日前に天気を予報するのは難しい」=「3日後の天気を予報するのは難しい」
ご参考まで!