一连, 频频, 再度: in a row, again and again, once more
【高20】【高21】【高27】副词 / adverbs
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
Three adverbs of repetition, all placed before the verb. 一连 describes an unbroken series and asks for a quantity after the verb: 一连下了九天雨, 一连打了三个电话. 频频 says that the same act or event comes back many times in quick succession: 频频点头, 频频举杯, 频频发生. No number follows it. 再度 means "once again, one more time" after an interval: 两位老同学三十年后再度相遇, 气象台再度发布预警.
Each has its own stress: 一连 underlines the lack of a pause, 频频 the frequency, 再度 the return. 再度 can speak of the past (再度夺冠) as well as of the future (将再度举行).
They are written words, typical of the press. In speech you would rather say 连续 or 一直 for 一连, 不停地 for 频频, and 又 (past) or 再 (future) for 再度 (hsk2-g59).
Examples
这场雨一连下了九天。
Zhè chǎng yǔ yìlián xià le jiǔ tiān.This rain fell for nine days in a row.
他一连打了三个电话，都没人接。
Tā yìlián dǎ le sān ge diànhuà, dōu méi rén jiē.He made three calls in a row and nobody picked up.
大家频频举杯，祝贺新馆开放。
Dàjiā pínpín jǔbēi, zhùhè xīnguǎn kāifàng.Everyone raised their glasses again and again to celebrate the new building's opening.
近来，这个路口频频出现交通事故，居民都很担心。
Jìnlái, zhège lùkǒu pínpín chūxiàn jiāotōng shìgù, jūmín dōu hěn dānxīn.Lately accidents have kept happening at this junction, and residents are worried.
两位老同学分别三十年后再度相遇，高兴得说不出话来。
Liǎng wèi lǎo tóngxué fēnbié sānshí nián hòu zàidù xiāngyù, gāoxìng de shuō bu chū huà lái.The two old classmates met again after thirty years apart, too happy to speak.
受冷空气影响，本市气温将再度下降。
Shòu lěngkōngqì yǐngxiǎng, běnshì qìwēn jiāng zàidù xiàjiàng.Because of cold air, temperatures in the city will fall again.
Common mistakes
✗ 他一连来了。 → ✓ 他一连来了三天。 (一连 asks for a duration or a number after the verb).
✗ 他频频点了三次头。 → ✓ 他频频点头。 (频频 already means "again and again": no number of times).
✗ 请你再度说一遍。 → ✓ 请你再说一遍。 (再度 is a written word that reports a fact; a spoken request takes 再).