三思而後行 Look before you leap. 
三句不離本行 to talk shop 
三個臭皮匠勝過一個諸葛亮 Two heads are better than one. 
三番兩次 This guy is at me(找我碴) time after time.(time and again; time and time again.) 
三緘其口 You'd better hold your peace, or you'll ask for trouble. 
千鈞一髮 to hang by a hair thread / at the critical time 
千錘百煉 A good nail does not fear the hammer. (steeled and tempered; gone through fire and water) 
千載難逢 It only happens once in a blue moon. (It chances in an hour that happens not in seven years.) 
千變萬化 The unexpected always happens. 
大刀闊斧 It's said that the new executive will go the whole hog (reform in a big way.) 
大同小異 The two articles are much of muchness ( be alike with minor difference ; There's not a pin to choose between the two ) in appearance and quality. 
大言不慚 You really blow your own trumpet. (talk big; overshoot oneself)