All but three of Prince George's high school seniors passed minimum reading standards required for graduation in Maryland last May, according to test data released yesterday. County 10th graders continued to read and do math at or near the national average skill.

The 1982 class was the first required to pass the Maryland Functional Reading test as a condition of graduation since testing began four years ago under Maryland law. The test is administered in the fall and spring of each year to all high school students who have not previously passed with a grade of 80 percent or better. In all, 8,695 county seniors had passed the test by last May. Three seniors who did not pass had not completed other course requirements for graduation, according to school officials, and they are still in school.

Almost 1,500 high schoolers out of more than 37,000 enrolled had not satisfied the requirement as of last May, but most of those were in the lower grades.

" Students say that this is just one more test and who cares," explained Louise Waynant, director of instruction for the county schools. "But you find that most seniors . . . take it very seriously."

The test is designed to measure "everyday reading, things we tend to read during the course of the year as citizens," Waynant said. It was born out of state legislative fears that Maryland schools might be turning out functional illiterates. Multiple choice questions are based on reading samples from traffic signs to magazine articles, Waynant said.

The Comprehensive Test of Basic Skills, the second set of results released yesterday, is a stricter measure of both reading and mathmatical ability, given only in Prince George's as a diagnostic tool. Overall the county 10th graders scored slightly above national norms in language skills for 10th grade and slightly below in reading and math