Two plants are crossed, resulting in offspring with a 3:1 ratio for a particular trait. what does this suggest?

This suggests that the trait observed is a single allele dominate trait and that both parents were heterogeneous in that they had a copy of both the dominate and the recessive allele. For the offspring, the 1/4 of the population that doesn't show the trait, all of them have the recessive allele. Of the 3/4ths that show the trait, 2/3rds of them will also be heterogeneous like their parents, and 1/3rd will be homogeneous in the dominant allele.