Experts the formula for 1st order reaction is derived from a single reactant forming multiple products and the formula is kt = ln [Ao]/[At] So my doubt is that here there are 3 reactants ester, h2o and acid then how is this formula applied at the end. And what does this formula represent here?

In ester hydrolysis reaction, acid acts as a catalyst, so its concentration is not considered. But the concentration of water is excess and so its considered as unity. 
Thus the rate of the reaction depends only on the concentration of ester. 
R = k [ester] [water]
R= k [ester]
Such reactions are called pseudo unimolecular reaction. The order of the reaction is 1. 

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