f(√2x+1))=2x-1=√2*√2x -1
=√2*√2x +√2 -1-√2
=√2(√2x+1)-1-√2
令√2x+1=x
∴f(x)=√2 x -1-√2
f(3)=3√2-1-√2
=2√2-1
选D