Open Placement Properties and go to the Label Density tab.
First, check the "remove duplicates" box and then open the Duplicate Labels Options window. Set the search radius to the minimum acceptable distance between labels (perhaps 2.5 inches in your case).
Second, check the "repeat label" box and then open the Label Repetition Options windows. Set the minimum repetition interval to to the maximum acceptable distance between labels (perhaps 3.5 inches in your case).
Third, ensure that the "connect features" box is checked, so that Maplex effectively views adjacent lines with the same name as the same line.
This will guarantee a label every 2.5" to 3.5" along roads with the same name.