This page should be renamed because, unlike "level" planets, "grade" planets seem to spell out the number.
Talk about it here.
Grade 3 planets, also written as Grade Three Planets, (PROSE: Doctor Who and the Ribos Operation) Grade Three planets, and grade three planets, were primitive worlds which were in a pre-space age state. The Central Galactic Census Bureau defined a grade three planet as "a planet that has no significant technology, no knowledge of other worlds, and as such, has a protected status". Strellin was considered grade 3, essentially a medieval society, where landings by offworlders were strictly prohibited. (AUDIO: Order of the Daleks)
Ribos was also considered grade 3, and would take thousands of years to reach grade 2 status. Until then, commercial mining could not take place as the inhabitants were unaware of other life forms. (TV: The Ribos Operation)