I tried to build a formula that went from your Total column to each of the other two, but the results were inconsistent. The closest I got was to assume amounts should be rounded to the nearest 5 cents.
The following formula assumes your sample data are in A2:D2. Values in D2 are given. The formulas for B2 and C2 (less 15% discount and plus 3% GST) are shown below:
The figures in your example table neither round up to the dollar nor to 10 cents. So I don't understand how to interpret "I required to derive to a nearest 0.10 cent if its above 0.5 should be 1.0 then."
Either revise your example table, or clarify how you want the calculations to be done.