You don't say how old you are but my parents 'kindly' presented me with a brother when I was 13 1/2 yes remember the half - I was shocked! We were a bit like 2 only children but I used to give him little things like a cool t-shirt & when he was a little bit older & a birthday pressie I bought was too big to wrap I made up a simple treasure hunt which he thought was cool. He's now in his forties & there is really no age gap now xxx

Spend time with him, dont try to discipline him or tell him what to do as this is his parents job and will confuse him. If you take the time to read to him and play with him he should learn to associate you with having fun. This may take time as three year olds can be very difficult!