Question

Two positions of a cube are as follows. 

Point out which number will be on the top if number 3 is at the bottom.

Answer 1

A cube has six faces which are denoted by dots and numbers.
They range  from 1 to 6. 
Most of the dices have got dots behind them while some have got numbers.
However, both use the same conventional method for marking the sides.
Generally,
1 is opposite to 6
2 is opposite to 5 and 
3 is opposite to 4
Hence when 3 is at the bottom, 4 will be at the top
4 is the answer

Answer 2

The answer is 4.

Let us consider the cube in question to be a dice. In a dice, the sum of the markings on two opposite sides is equal to seven markings. In most cases, the markings are dots. However, the dots are replaced by numbers in some dies. In this case, the sum of the digits on two opposite faces will be seven. Thus,

If the dice has 1 at the top face, the bottom face will be 6 and vice versa.

If the dice has 2 at the top face, the bottom face will be 5 and vice versa.

If the dice has 3 at the top face, the bottom face will be 4 and vice versa.