\[\boxed{\mathbf{3.}}\]
\[\textbf{а)}\ f\left( g\left( \varphi(x) \right) \right) = \sin\sqrt{x^{3}};\]
\[f(x) = \sin x;\]
\[g(x) = \sqrt{x};\]
\[\varphi(x) = x^{3}.\]
\[\textbf{б)}\ f\left( g\left( \varphi(x) \right) \right) = \left( \sqrt{\sin x} \right)^{3};\]
\[f(x) = x^{3};\ \ \]
\[g(x) = \sqrt{x};\]
\[\varphi(x) = \sin x.\]