The problem provides not enough information, hence, supposing that you need to write the equation of amplitude 3 of an wave, you need to use the following equation, such that:
`y = A*sin omega*t`
`A` represents the amplitude
`omega` represents the frequency
`t` represents the time period
Since you only need the amplitude of 3, you should replace it in equation above, such that:
`y = 3sin omega t`
Hence, you may write an equation of amplitude 3 of an wave, such that `y = 3sin omega t` .