The child suffered serious burns to his arms and face after being targeted at Home Bargains on Shrub Hill Retail Park at around 2.15pm on Saturday.
He is being treated for his injuries in hospital.
West Mercia Police have released images of three men they would like to speak to about the incident.
Chief Superintendent Mark Travis said: “At this time we are treating this as a deliberate attack on a three-year-old boy.
“The incident will rightly shock the local community and I would like to reassure local people that we are carrying out a thorough investigation to identify those responsible.
“At this time, the motive for the attack is unclear. Officers are working hard to establish exactly what happened and what the substance involved was.”
He said that the three men pictured in the images from the store “may have information vital to our investigation.”
“I am sure someone will recognise them and I…