Your initial noise is uniformly distributed between -1 & +1
Your second noise is also uniformly disributed between -1 & +1, because mean is already zero, subtracting it is meaningless
in order to obtain white noise you can use randn()
function:
wn = randn(length(t),1); %zero mean variance 1
You may not observe any much difference again if your noise coefficient A3 has a much lower value compared to 20 & 30 which are the coefficients of your signal.
In order to find peaks, adding noise may not serve any purpose because noise tends to decrease the information content of signals