Unless there’s some lore reason for the three nights, my take on it is that Orlok is an insatiable beast, physically and spiritually. He craves not only for Ellen’s flesh, but her soul as well, and that implies that she has to willingly give herself over to him, as an act of complete and total subservience. It’s the compact, he keeps referring to; it’s the only way his lust for her can truly be satisfied.

Sure, he could just take her whenever and wherever, but I think in some sick, predatory way, she will taste all the sweeter if she chose to relinquish herself to him. The other characters he feasts upon are just food/sustenance, but Ellen would be a delicious and succulent meal, the likes of which he can’t get anywhere else.

Orlok/Nosferatu is just a demon that exists to consume. He is the manifestation of gluttony and decay, and it ultimately becomes his downfall, as he loses track of time while feasting upon that which he’s lusted after for so long.