The suspects, aged 22, 25 and 26, were detained in London on suspicion of conspiracy to commit grievous bodily harm, West Mercia Police said.
A 39-year-old man from Wolverhampton arrested on Sunday on suspicion of conspiracy to commit grievous bodily harm remains in custody, the force said.
The toddler was in a pushchair with his family in a shop when a corrosive substance was '˜thrown or sprayed' over him on Saturday afternoon, according to West Mercia Police.
He was treated in hospital for burns to his face and one arm before being released on Sunday afternoon.
Police said the long-term implications of his injuries are '˜uncertain'.
Worcester City Council leader Marc Bayliss described the attack as '˜absolutely pure evil'.
Worcester MP Robin Walker said the attack was '˜horrific', and that the '˜shock will be universal'.
Police are continuing to appeal for information over the incident at Home Bargains in Tallow Hill.
Detective Inspector Tony Garner said: '˜We'd like to thank everyone who shared our appeal over the weekend and contacted us with information; all of this is helping us to build up a better picture of this incident.
'˜We're continuing to urge anyone else with information to contact us.'
Anyone with information that could help police with their enquiries was asked to call 101 quoting incident 442s of 21 July 2018.
Alternatively, information can be provided anonymously to the independent charity Crimestoppers on 0800 555 111.