The second gives $$x=a^{\frac{1}{b}}$$ because there is the following property.
$$\log_{a^{\alpha}}b=\frac{1}{\alpha}\log_ab,$$ where $\alpha\neq0,$$a>0$, $a\neq1$ and $b>0$.
Because for $x=a^{\frac{1}{b}}$ we obtain:$$\log_xa=\log_{a^{\frac{1}{b}}}a=\frac{1}{\frac{1}{b}}\log_aa=b.$$
The second gives $$x=a^{\frac{1}{b}}$$ because there is the following property. $$\log_{a^{\alpha}}b=\frac{1}{\alpha}\log_ab,$$ where $\alpha\neq0,$ $a>0$, $a\neq1$ and $b>0$.
Because for $x=a^{\frac{1}{b}}$ we obtain:$$\log_xa=\log_{a^{\frac{1}{b}}}a=\frac{1}{\frac{1}{b}}\log_aa=b.$$
The third gives $$x=\log_ab$$