All of the four functions are exponential functions of the form:
Where,
P changes the parent function's, , slope (more steep or less steep)
Q shifts the parent function, , upwards if Q is positive and downwards if Q is negative.
The parent function looks like the first image attached. Note that the x-axis (y=0) is the horizontal asymptote.
Since question asked for a function with horizontal asymptote of y=3, we can shift the function 3 units above by changing Q to +3. The fourth option has an equation of an exponential that has Q value of +3. This is the correct choice.