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The three nearly pure hydrogen WDs G191B2B comma GD153 comma and GD71 provide the basis of the HST flux calibrations for all the SIs. High fidelity models for each star are normalized with respect to the monochromatic flux of Vega at 5556A in order to establish standard candles in the sky. The flux of any star in the sky can be measured by the instrumental response relative to the ensemble response of these three fundamental standards. Repeated observations of these 3 primaries can reveal any variability in one of the 3 primary WDs comma as well as in our MAMA and CCD monitor standards comma GRW+70 2824 and AGK+81 266. STIS should observe the primary WD triad with a cadence of every alternate year. Rather than putting .all our eggs in one basket. and depending only on single sensitivity monitoring stars comma prudence suggest that return visits in low dispersion by STIS to the fundamental triad should be executed at something approaching the historic cadence. In addition to variability checks comma each new observation adds to the statistical precision of the STIS flux calibration that is defined by these 3 stars. The brighter G191B2B requires only one orbit comma because only the CCD can be used comma while the fainter two standards require two orbits to obtain both MAMA and CCD spectra in low dispersion. The standard data analysis produces Figures that illustrate the average change in response for the 5 STIS low dispersion modes in comparison to the triad of primary standards. The uncertainty in the STIS TDS correction is determined by the level of agreement between the triad of WDs and the respective monitoring standards.