The victim was in a pushchair when a substance was sprayed or thrown towards him as he was deliberately targeted.
He was treated in hospital after suffering serious burns to his face and an arm, and has now been released.
Police have said the long-term medical implications for the child are unknown but detectives hope he will not have life-changing injuries.
The attack happened around 2.15pm on Saturday as the boy was on a shopping trip with his family in Home Bargains at Shrub Hill Retail Park, Tallow Hill.
West Mercia Police have released images of three men they would like to speak to and have appealed for anyone with information to come forward.
The men had been in the store and were seen walking close to the little boy. They left shortly after the assault.
Chief Superintendent Mark Travis said At this time we are treating this as a deliberate attack on a three-year-old boy.
He told Sky News It’s an awful incident and we’re really, really concerned about this.
Worcester is a really, really safe place. It’s very unusual to have any form of attack like this.
He said the incident will rightly shock the local community and that police were carrying out a thorough investigation to identify those responsible.
The motive of the attack is unknown.
An ambulance arrived at the scene in eight minutes and crew assessed the boy’s injuries. He was treated at the scene and then taken to hospital in Worcester.
His burns were consistent with an acid or alkaline substance, said Mr Travis.
A 39-year-old man from Wolverhampton has been arrested on suspicion of conspiracy to commit grievous bodily harm.
It is not known if he is connected to the three men being sought.
The three men in the photo may have information vital to our investigation, Mr Travis added.
I am sure someone will recognise them and I would urge anyone with information to contact us as a matter of urgency.
Worcester City Council leader Marc Bayliss described the attack as absolutely pure evil.
He added Worcester is not that sort of place. We are a quintessential small English city.
I have never heard of an acid attack in Worcester so this is absolutely not something we have any experience of.
Mr Bayliss urged anyone with information to come forward.
Think if this was a member of your family, an innocent child, a three-year-old probably scarred or damaged for life by this. We need to bring the perpetrators to justice, and quickly.
A Home Bargains spokeswoman said Our thoughts are with this young child and his family at this very difficult time.
Anyone with information should call 999 quoting incident 442s of 21 July 2018. Alternatively, call Crimestoppers anonymously on 0800 555 111.
News Source SkyNews