Cháng cháng (常常), shí cháng (時常) and jīng cháng (經常) all could be used to express frequency as adverbs, meaning “often” or “frequently”.They are interchangeable in most cases.
他最近工作很忙, 常常/經常/時常加班到深夜。
tā zuì jìn gōng zuò hěn máng, cháng cháng /jīng cháng /shí cháng jiā bān dào shēn yè.
He is very busy recently and often works late into the night.
他最近工作很忙, 常常/經常/時常negation [不吃早飯就去上班]。
tā zuì jìn gōng zuò hěn máng, cháng cháng /jīng cháng /shí cháng bù chī zǎo fàn jiù qù shàng bān.
He is very busy recently and often goes to work without eating breakfast.
Although not so commonly used, jīng cháng(經常) can also serve as an adjective, in this case, it can be modified by other adverbs like the negative adverb bù (不). Cháng cháng(常常) and shí cháng(時常) can’t be used this way.
他遲到是經常的事情。
tā chí dào shì jīng cháng de shì qíng.
It's common for him to be late.
我不negation[經常去電影院]。
wǒ bù jīng cháng qù diàn yǐng yuan.
I don't often go to the cinema.
