Simple diffusion is the movement of molecules (usually small, uncharged and non-polar) along a concentration gradient from their region of higher concentration to a region of their lower concentration. It does not require ATP as it is moving along the concentration gradient.
Facilitated diffusion is the diffusion of those molecules which cannot generally pass through the cell membrane because of their size, charge, polarity etc. so they are passed through the membrane aided by channel proteins and carrier proteins which are specific for certain molecules. It also does not require energy because the movement of molecules is along the concentration gradient.
Active transport is the movement of molecules against their concentration gradienti.e from region of their lower concentration to region of their higher concentration with help of some energy. Hence ATP is required and it is an uphill transport that is highly selective allowing transport for a specific group of molecules.
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