idk about guitar or violin, but piano is def the most difficult, cuz u have to read 2 clefs, treble and base (like if ur reading a book, you would be reading 2 lines at a time) and both hands are doing completly different things, one is harmony and the other is melody. idk if u already know how to read music, but if you do it will def make learning piano easier. I play piano and clarinet, and clarinet is way easier than piano in terms of moving fingers/hands cuz there's only one clef to read