1. Yes You Can use existing universal LNB to receive signals from KU band From Any Satellite,
2. Existing STB Means Which Given With Any DTH Service Providers Or You Bought Separately?
If You Have Any Existing FTA STB You Can Use That To Watch FTA Channels, If It Is Mpeg4 Then It'll Good, You'll Get FTA HD Channels Too
3. It Is Impossible To Receive C-Band Signal Of Asiasat 3S In D2H Dish, You Need Minimum 6 Feet Dish For That