“闺蜜”、“小三”、“有外遇”，用英语怎么说？

1

bestie 闺蜜

I regarded her as my bestie but it turns out that I am just her ordinary friend.

我拿她当闺蜜，但是对于她来说我只是个普通朋友

2

mistress 小三

I even can't believe that he has a mistress till now.

我直到现在都不敢相信他有小三

3

have an affair 有外遇

My boyfriend has an affair with my bestie, what a ridiculous world!!

我男朋友跟我闺蜜出轨了，这世界太荒唐了