As adverbs, 常常，经常 and 时常 all indicate there is something occurred more than once, and it occurs repeatedly. Sometimes, they are interchangeable.
我们常常/经常/时常一起讨论数学。(Wǒmen chángcháng/jīngcháng/shícháng yīqǐ tǎolùn shùxué.) We often discuss mathematics together.
这里的夏天常常/经常/时常有大风。(Zhèlǐ de xiàtiān chángcháng/jīngcháng/shícháng yǒu dàfēng.) There are often strong winds here in summer.
这个学生常常/经常/时常被表扬。(Zhège xuéshēng chángcháng/jīngcháng/shícháng bèi biǎoyáng.) This student is often praised.
1. 经常 emphasizes the same action or situation that occurs repeatedly and constantly. There is consistency.
屋子要经常打扫才干净。(Wūzi yào jīngcháng dǎsǎo cái gānjìng.) The house must be cleaned frequently.
他上班经常迟到。(Tā shàngbān jīngcháng chídào.) He is often late for work.
常常 emphasizes the many times the occurring of an action or behaviour.
我以后会常常给你打电话的。(Wǒ yǐhòu huì chángcháng gěi nǐ dǎ diànhuà de.) I will call you often in the future.
他会常常来看你的。(Tā huì chángcháng lái kàn nǐ de.) He will come to see you often.
时常 emphasizes the action or situation occurs more than once, and it is not necessarily high frequency.
他身体不好，时常生病。(Tā shēntǐ bù hǎo, shícháng shēngbìng.) He is in poor health and often gets sick.
这条街时常有小偷。(Zhè tiáo jiē shícháng yǒu xiǎotōu.) There are often thieves in this street.
2. If there is a word like “坚持” “保持” in a sentence, 经常 can be used, but 常常 or 时常.
我的家人经常坚持运动。(Wǒ de jiārén jīngcháng jiānchí yùndòng.) My family always keeps on exercising.
大家应该经常保持联系。(Dàjiā yīnggāi jīngcháng bǎochí liánxì.) Everyone should keep in touch frequently.
3. When functioning as an adverbial, 经常 can be followed by 地, but 常常 and 时常 cannot.
√ 我经常(地)晚睡早起。(Wǒ jīngcháng de wǎn shuì zǎoqǐ.) I often go to bed late and get up early.
✘ 我常常/时常地晚睡早起。(Wǒ chángcháng/shícháng de wǎn shuì zǎoqǐ.)
√ 他上课经常(地)迟到。(Tā shàngkè jīngcháng de chídào.) He is often late for class.
✘ 他上课常常/时常地迟到。(Tā shàngkè chángcháng/shícháng de chídào.)
4. 经常 can also be an adjective, while 常常 and 时常 cannot.
他工作很忙，没时间吃饭是经常的事。(Tā gōngzuò hěn máng, méi shíjiān chīfàn shì jīngcháng de shì.) He is very busy at work and it is common for him to have no time to eat.
他睡得很晚，所以早上起得晚是经常的。(Tā shuì de hěn wǎn, suǒyǐ zǎoshang qǐ de wǎn shì jīngcháng de.) He sleeps very late, so it is common to get up late in the morning.
5. 时常, 经常 and 常常 can share the same negative form 不常. We can also use 不 to negate both 经常 and 常常, although 不常常 is not super common, while 时常 cannot.
√ 我跟她不经常/常常/常联系。(Wǒ gēn tā bù jīngcháng/chángcháng/cháng liánxì.) I don’t contact her often.
✘ 我跟她不时常联系。(Wǒ gēn tā bù shushing liánxì.)
√ 这个公司不经常/常常/常开会。(Zhège gōngsī bù jīngcháng/chángcháng/cháng kāihuì.) This company does not hold meetings often.
✘ 这个公司不时常开会。(Zhège gōngsī bù shícháng kāihuì.)
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