我要和外国人说叫他先成地铁到上海南站,然后换三号线,英语怎么说啊

you may take line 1 to 上海南站 and interchange line 3

上面的翻译基本正确，只是transfer这里用作不及物动词。
Take the Metro (subway) to Shanghai South Railway Station, where you get off and transfer to line 3.

Take the Metro (subway) to Shanghai South Railway Station, where you get off and transfer line 3