it , that が示すもの

下線部のit,thatは何を表してますか？文中から抜き出してほしいです
1)It makes me happy to see people give up their seat to an elderly person or a pregnant lady. I have seen 2)that happen in trains or buses many times and I have always felt good to see that happen. Of course I also do 3)that whenever I have a chance.