The expression "a pair of genes" can refer to any two genes of the genome present either on the same chromosome, on homologous chromosomes or on two non-homologous chromosomes. Hence, this expression cannot semantically refer to one gene. However, "a pair of alleles" does semantically refer to a single gene, since the term's definition itself is "the variations possible of a single gene".
The expected ratio of 3:1 typically occurs while dealing with two alleles of contrasting characters with one being dominant over the other in a single cross (monohybrid) of two homogeneous parents, i.e., homogeneous dominant parent crossed with homogeneous recessive parent for a single trait. In this cross, the result is that the 1st generation has the ratio dominant:recessive = 3:1.
I am not completely familiar with the trait of rust resistance and its inheritance patters, but I found one research paper (whose link is given below) which states that inheritance of rust resistance in wheat is generally dominant, although most research states that this pattern varies with each plant/strain.
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