If $3$ mangoes are sold for the cost price of $4$ mangoes, then the profit is:

Let the Cost Price (CP) of $1$ mango be $\text{Rs. }1$.
Therefore:

* CP of $4$ mangoes = $\text{Rs. }4$
* CP of $3$ mangoes = $\text{Rs. }3$

According to the question, the Selling Price (SP) of $3$ mangoes equals the CP of $4$ mangoes:

$$\text{SP of 3 mangoes}=\text{Rs. }4$$

Now, calculate profit and profit percentage on those $3$ items:

$$\text{Profit}=\text{SP}-\text{CP}=4-3=\text{Rs. }1$$

$$\text{Profit Percentage}=\left(\frac{\text{Profit}}{\text{CP of 3 mangoes}}\right)\times 100$$

$$\text{Profit Percentage}=\frac{1}{3}\times 100=\frac{100}{3}\%=33\frac{1}{3}\%$$