Some people say that those who beat their chest hard are doing self harm, and instead of chest beating the should show their love for Imam Hussain by fasting for 3 days, what can we reply to this?

It is not prohibited to beat the chest, even in a strong manner, out of sorrow for Imam Hussain (p). We do a lot of things that are potentially harmful to the body (overeating, not exercising), but it is not a violation of the religion because it does not rise to the level of severe harm. Thus, although it’s good to fast, performing matam is ok too. Each person expresses their own way of love for Imam, as long as it’s not haram we should respect them and not compare one to another