SFF-8301 specification from the industry-standard Small Form Factor (SFF) committee defines the dimensions for 3.5 inch Form Factor Disk Drives.
SFF-8301 defines three pairs of bottom mounting holes as follows:
Source: SFF Committee, SFF-8301 Specification for Form Factor of 3.5" Disk Drives Rev 1.6
March 16, 2010
"The pair of bottom mounting holes located by dimension A7 is required. One additional pair of bottom mounting holes are required, either the pair of mounting holes located by dimension A6 or the pair of mounting holes located by dimension A13. Providing all three pairs of mounting holes (located by dimensions A7, A6 and A13) is allowed."