Piero C. answered 03/16/19
Certified Teacher with 35 years of experience in Italy and abroad
They basically the same.
SOLO is used in a more colloquial way and can used both as an adjective and as an adverb. Notice the difference:
(ADJ: parlo un solo minuto)
(ADV: parlo solo un minuto; vorrei solo vederti)
SOLAMENTE and SOLTANTO can be only used as adverbs:
ADV: ( parlo soltanto un minuto; parlo solamente un minuto / vorrei soltanto vederti; vorrei solamente vederti)