Translation:Three youths were caught with a CCTV camera in a shop.
Llanc is the singular =boy or youth
Llanciau is the plural = boys, youths
Because numbers in Welsh take the singular noun to say 'three youths' we write 'tri llanc'
Therefore 'llanc' is the singular noun and cannot mean the plural on its own.
Geiriadur yr academi, the most comprehensive online English-Welsh dictionary gives a different definition of youngster:- 'rhywun ifanc, plentyn'