Perhaps, Someday by Edward Kendrick

Caiden McNeil is a special being, a metamorph who can become any human necessary to stop the criminals his elven boss Zander sends him after.Caiden is also a loner, unable to trust anyone on an emotional level. He has reason to feel this way: in his over eight-hundred years, he has learned that caring for someone only leads to despair. After all, how could a human accept him for what he really is, an immortal who will never seem to grow older than his apparent age of twenty-eight?Then two men come into Caiden's life — Leander, a cougar shifter, and Theirn, a half-elf. Either may betray Caiden and his belief that their feelings for him are real. On the other hand, is one of them willing to fight to prove love is possible, if Caiden will only give it a chance?