Hi!

I've watched the harmonizing video but can you tell me a harmonizing exercise because when I try to harmonize 1-2-3 to 3-4-5 the piano is stronger than my voice so it makes me change to the piano pitch.  

have u started ur lessons

I'd suggest singing 3-4-5 until you're confident with that; then add the piano (playing 1-2-3), but make sure the piano is very soft so it isn't overpowering. Hope that helps!