Defining datums from a pattern of features can be confusing and there are many ways it can be done. On the part shown, it may be desirable to use two of the holes in the three-hole pattern as datum features. One approach would be to apply individual controls to each of the holes in the pattern as the first drawing illustrates. Another approach is to control the holes as a pattern using position relative to the primary datum, A. Then, label individual holes as datum features as shown in the second plan view. Depending on the tolerances and modifiers used, nearly the same control may be achieved with two less geometric controls, which simplifies the drawing and may simplify verification while meeting functional requirements.

Note: the drawing below is incomplete by intent.