Is that "3MB" FORMATTED, or UNFORMATTED capacity?
"10MB" (ST412) was 12MB unformatted, with 306 cylinders and 4 heads."5MB" (ST406) was 6MB unformatted, with 306 cylinders and 2 heads. or was it (St506?) 153 cylinders with 4 heads?
In any case, using half as many heads/surfaces would create an "ST503" drive. about 2.5M formatted capacity.
