Identify change in oxidation state of oxidising agent in following redox reaction.
$3\text{H}_3\text{AsO}_{3(aq)} + \text{BrO}_{3(aq)}^- \longrightarrow \text{Br}_{(aq)}^- + 3\text{H}_3\text{AsO}_{4(aq)}$

The given reaction is $3\text{H}_3\text{AsO}_{3(aq)} + \text{BrO}_{3(aq)}^- \longrightarrow \text{Br}_{(aq)}^- + 3\text{H}_3\text{AsO}_{4(aq)}$

Let the oxidation state of As in $\text{H}_3\text{AsO}_3$ be $x$.
$3(+1) + x + 3(-2) = 0 \Rightarrow x = +3$

Let the oxidation state of Br in $\text{BrO}_3^-$ be $y$.
$y + 3(-2) = -1 \Rightarrow y = +5$

Let the oxidation state of As in $\text{H}_3\text{AsO}_4$ be $z$.
$3(+1) + z + 4(-2) = 0 \Rightarrow z = +5$

The oxidation state of Br in $\text{Br}^-$ is $-1$.

The oxidation state of As increases from $+3$ to $+5$, so $\text{H}_3\text{AsO}_3$ undergoes oxidation and acts as a reducing agent.

The oxidation state of Br decreases from $+5$ to $-1$, so $\text{BrO}_3^-$ undergoes reduction and acts as an oxidising agent.

The change in oxidation state of the oxidising agent is $+5$ to $-1$.

Answer: $+5\ \text{to}\ -1$