这三名警察都是美国人，他们都会说汉语

All of（注意这个不能少！） the three policemen are Americans, all of whom can speak Chinese.
或
All the three policemen are Americans, and all of them can speak Chinese.

All three are Americans, they will say Chinese

??????????

shen me....what

these three cops who are americans can speak chinese.

All the three policemen are Americans, all of whom can speak Chinese.

这三名警察都是美国人，他们都会说汉语
These three policemen are all American and each of them can speak Chinese.