He is being treated in hospital for serious burns to his arm and face.
The toddler was “deliberately targeted” in the attack which occurred at around 2.15pm on Saturday in Home Bargains on Shrub Hill Retail Park, Tallow Hill, officers said.
West Mercia Police have released images of three men they would like to speak to and have appealed for anyone with information 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.”
He said the incident will “rightly shock the local community” and that police are carrying out a thorough investigation to identify those responsible.
The motive of the attack is unclear at this time.
Officers are working to establish exactly what happened and what the substance involved was.
A 39-year-old man from Wolverhampton has been arrested on suspicion of conspiracy to commit grievous bodily harm and remains in police custody.
“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.”
Anyone with information that could help police with their enquiries should call 999 quoting incident 442s of 21 July 2018. Alternatively, information can be provided anonymously to the independent charity Crimestoppers on 0800 555 111.