One of the Wiimotes is a 6DOF headtracker. It tracks the position and orientation of your head as an input. This allows you to look, turn, lean, crouch, and jump without the use of buttons, joysticks, or pointers. The other two Wiimotes are in your hands.
Although you will have the option to play with two Wiimotes total (one as a pointer and the other as a headtracker) the gameplay is as crippled with wiimote alone or wiimote+nunchuck as you personally would be if you went through life one-handed. Imagine a corridor with no floor and rungs on the ceiling which are too far apart to reach with your hands normally. Swing like Tarzan. You "grab" on one from far away and swing. What about the next rung though? You don't want to very well let go of your current rung to grab the next one, so you grab it with your other hand and keep swinging. Starting to see now why you need two hands (also known as "Twomote") which can point independently from the screen?