The boy is being treated for "serious burns to his arm and face" in hospital following the mid-afternoon incident at a discount store in the city of Worcester, according to local police.
A 39-year-old man from the nearby city of Wolverhampton was arrested on suspicion of conspiracy to commit grievous bodily harm and remained in custody Sunday, it said.
Police also released photos of three men that investigators would like to speak to.
"At this time we are treating this as a deliberate attack on a three-year-old boy," chief superintendent Mark Travis said, adding the motive was unclear.
"Officers are working hard to establish exactly what happened and what the substance involved was.
"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," he added.
Britain has seen a spike in the number of acid attacks in recent years, with hundreds of incidents in London alone every year.
The trend has prompted government intervention initiatives while police have asked the public for help to try to tackle the problem.
Published : July 23, 2018
By : Agence France-Presse London