The rule of thirds is$...$

a process recognizing that funding will be split into three groups with differing sizes of groups and expectations for levels of giving.

a process of separating all potential donors into three equal groups with the emphasis on securing one$-$third of the dollars from each group

a recognition that only one$-$third of the money actually desired will be secured through fund raising

an effort to encourage fund raisers working for you to secure funding by pitting three different groups against each other