A corn inbred designated 3B603 is disclosed. The invention relates to the seeds of corn inbred 3B603, to the plants of corn inbred 3B603, to plant parts of corn inbred 3B603 and to methods for producing a corn plant produced by crossing corn inbred 3B603 with itself or with another corn variety. The invention also relates to methods for producing a corn plant containing in its genetic material one or more transgenes and to the transgenic corn plants and plant parts produced by those methods. This invention also relates to corn inbreds or breeding inbreds and plant parts derived from corn variety 3B603, to methods for producing other corn inbreds and lines or plant parts derived from corn inbred 3B603 and to the corn plants, varieties, and their parts derived from use of those methods. The invention further relates to hybrid corn seeds, plants and plant parts produced by crossing the inbred 3B603 with another corn inbred.