It may be pot bound. Alstroemeria (these are not really any kind of lily....not even a bulb!) naturalize or spread via very fleshy, tuberous roots. They need some room to spread to do well. Three years in the same pot is likely two years too long :-)) Larger pot and fresh potting soil. And these container grown plants will need routine fertilizing as well. Any sort of liquid fertilizer designed to encourage flowering (ie, Bloom Booster, Mor-Bloom, etc.) will work perfectly well.