三心两意
in two minds
My wife thinks we should go to South Korea for our holiday, but I'm still in two minds about it.

 
三番两次
again and again; over and over (again); time and (time) again; time after time
I've told my son again and again not to run a red light. (to run a red light：闯红灯)

 
三缄其口
hold one's peace/tongue
In spite of the fact that he made provocative remarks, I held my peace/tongue.

 
三思而后行
look before you leap
Look before you leap when doing business on the Web.

 
三句不离本行；说行话；谈论工作上的事情 (这成语通常带有贬义，暗示这些关于工作的言谈让人觉得很无聊)
(to) talk shop
They did nothing but talked shop, and I came home early because it was so boring.

三个臭皮匠顶个诸葛亮
two heads are better than one
Two heads are better than one.

 
大同小异；半斤八两
(very) much of a muchness
The two articles are very much of a muchness.

 
大相径庭
poles apart
They are poles apart in their political attitudes.

 
千方百计
by hook or by crook; leave no stone unturned (or no stone be left unturned)
I know this work looks difficult, but I'll finish it by hook or by crook.
The police left no stone unturned to catch Taiwan's most wanted fugitive Chang Hsi-ming (张锡铭) and they succeeded.
He promised no stone would be left unturned to get his wife back.

千载难逢
once in a blue moon
My wife and I rarely, if ever, eat out. So, when I told her last night "let's eat out and go to see a movie", she was very surprised and said that's really once in a blue moon.

寸步不离
at one's elbow
His wife was always at his elbow whenever an important decision had to be taken.

大海捞针
(like) looking/searching for a needle in a haystack
Trying to find a book we want in so many books is just like looking for a needle in a haystack.

 
口是心非；言不由衷
with one's tongue in one's cheek (adv.); tongue in cheek (adv.); tongue-in-check (adj.)
He described me as an excellent teacher, but he said it tongue in cheek/with his tongue in his cheek.
That's a tongue-in-cheek remark/reply.

大惊小怪；小题大作
make a mountain out of a molehill
Don't make a mountain out of a molehill when you know I look about ten years younger than I really am. (I look about ten years younger than I really am. 我看起来比实际年龄年轻大约10岁)

 
小题大作；杀鸡用牛刀
break a butterfly on a/the wheel (「小题大作」亦可用 "make a meal out of" 和 "make a mountain out of a molehill" 来表示)
I know what I've done is wrong, but there's no need to break a butterfly on a wheel/to make a meal out of it.

大发雷霆
do one's nut
When I told him what she had said about him, he did his nut.

 
大获全胜；所向披靡
sweep the board; carry (or sweep)  all/everything before one
I swept the board at the casino last night.
Although I hadn't done well at high school, I carried all before me at university.

亡羊补牢
lock the stable door after the horse has bolted
It is never too late to lock the stable door after the horse has bolted.

小洞不补，大洞吃苦
a stitch in time saves nine (经常略为 "a stitch in time")
You should try and repair your car before it becomes worse. Remember, a stitch in time saves nine.