In humans and other diploid organisms which have two sets of chromosomes, one individual can have two copies of the same allele or two different alleles. Locus refers to a location on the chromosome or genetic map.
Gene refers to the genetic information or the DNA sequence at that position.
Trait refers to the manifestation or phenotype of the particular gene. In practice, gene and locus are often used interchangeably. The gene for eye color is at the eye color locus. Allele is clearly distinct, referring to a specific form of the gene. Both chromosomes of a pair have the same loci all the way along their length, but may have different alleles at some of the loci.