Set signal level to -3dB

I have a sound, and from that sound a get a signal form and spectrum and i need to set this sound signal level to -3dB how to do that.

if true
  % code
endclear all; close all; clc % 
[y,Fs] = audioread('Kulka.wav');
s = y(:,1); 
N = length(s); 
n = 1:N ; % imtis
S=fft(s); % signalo FFT
Faxis=(n-1)*Fs/N; % dazniu asis
Sa = abs(S)/(N/2); % amplitude
figure(1) % FFT atvaizdavimas
plot(Faxis, Sa)
xlabel('Daznis, Hz');
ylabel('Amplitude, V');
title('FFT');
xlim ([0 Fs/2]); grid on
nfft = 1024; % spektrogramos pradzia
window = nfft;
noverlap = window-10;
sound(s,44100); %garso atkurimas
figure(2) % Spektrogamos atvaizdavimas
specgram(s,nfft,Fs,window,noverlap) % Spektrograma
xlabel('Laikas, s'); 
ylabel('Daznis, Hz');
title('Spektrograma');