Attackers were seen on CCTV throwing objects at the animals and chasing them at Danesmoor Nursing Home, Haslingden, Lancashire, on Tuesday night.
The alpacas - named Bill and Ben - are brothers and have lived at the home for five years.  They were not hurt.
Lancashire Police said two 17-year-olds and a 16-year-old voluntarily attended Waterfoot police station on Thursday.
A spokesman said they were spoken to "in relation to offences of animal cruelty and criminal damage".
He added: "They have now been referred on to the youth offending team."
Three youths have been spoken to by police over an attack on two alpacas with garden furniture.