Step 1: Understand the condition for precipitation.
For precipitation of silver halide,
\[
AgX(s)
\]
the ionic product must exceed the solubility product.
The precipitation starts when
\[
[Ag^+][X^-]=K_{sp}
\]
Therefore, the required concentration of \(Ag^+\) for precipitation is
\[
[Ag^+]=\frac{K_{sp}}{[X^-]}
\]
Step 2: Note the halide ion concentrations.
The given concentrations are
\[
[Cl^-]=[Br^-]=[I^-]=1\times 10^{-8}\ M
\]
Since all halide ion concentrations are equal, the salt with the smallest \(K_{sp}\) will require the least \([Ag^+]\) and will precipitate first.
Step 3: Compare the solubility products.
Given,
\[
K_{sp}(AgCl)=1.8\times 10^{-10}
\]
\[
K_{sp}(AgBr)=5\times 10^{-13}
\]
\[
K_{sp}(AgI)=8.3\times 10^{-17}
\]
The order of \(K_{sp}\) values is
\[
AgI<AgBr<AgCl
\]
Step 4: Determine the precipitation order.
Lower \(K_{sp}\) means lower solubility and earlier precipitation.
Therefore, the order of precipitation is
\[
AgI,\ AgBr,\ AgCl
\]
Step 5: Final conclusion.
Hence,
\[
\boxed{AgI,\ AgBr,\ AgCl}
\]
Therefore, the correct option is (3).