Question

Why do we take three or more readings and then find the mean value when we want to find the weight or volume of an object?
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Answer 1

It is done inorder to get more accurate result (approximate to correct result)
eg:
Let's take an example. Suppose you want to find the mass of a gold ring that you would like to sell to a friend. You do not want to jeopardize your friendship, so you want to get an accurate mass of the ring in order to charge a fair market price. You estimate the mass to be between 10 and 20 grams from how heavy it feels in your hand, but this is not a very precise estimate. After some searching, you find an electronic balance that gives a mass reading of 17.43 grams.So we cannot be assure that its mass is 17.43grams.When you use the same electronic balance and obtain several more readings: 17.46 g, 17.42 g, 17.44 g,.

So if we take the mean of these measurements you will get most accurate anwer to confidently tell your friend the mass of gold ring.

For further information which you will learn in higher classes,
This is actually in accordance with 2 terms of measurements accuracy and precision:
Accuracy is the closeness of agreement between a measured value and a true or accepted value. Measurement error is the amount of inaccuracy.
Precision is a measure of how well a result can be determined (without reference to a theoretical or true value). It is the degree of consistency and agreement among independent measurements of the same quantity; also the reliability or reproducibility of the result.
So For more precise and accurate measurement we take three or more readings and then find the mean value when we want to find the weight or volume of an object