Cr + 3fe3+cr3+ + 3fe2+ in the above redox reaction, use oxidation numbers to identify the element oxidized, the element reduced, the oxidizing agent and the reducing agent.

Element Cr goes from Cr --> Cr³⁺
element Fe goes from Fe³⁺ --> Fe²⁺
oxidation reactions are when the species gives out electrons and its oxidation number increases.
reduction reactions are when the species take in electrons and its oxidation number reduces.
Cr : oxidation number of Cr - 0 and Cr³⁺ - +3
Theres an increase in oxidation number from 0 to +3. Therefore its an oxidation reaction. Cr gets oxidised. 
Fe : oxidation number of Fe³⁺ - +3 and Fe²⁺ - +2
There's a decrease in oxidation number of Fe from +3 to +2, therefore its a reduction reaction and Fe gets reduced.
         
Cr reduces Fe - Cr is the reducing agent 
Fe oxidises Cr - Fe is the oxidising agent