C'è differenza tra «solo», «solamente», e «soltanto»?
First of all, I think it would be useful for you to learn the differences between adjectives, adverbs, nouns, prepositions and conjunctions - not only just for Italian. It will help clear up future confusions like these.
From my understanding, "solo", "solamente" and "soltanto" are conjunctions/adverbs that mean something like "only" or "just". However, among these three, "solo" is the only one that can be used as a noun or an adjective. As a noun, it means "single person/thing" and when used as an adjective, it means "alone".
What said lasoufriere is all right, I would add some examples. "Solo", "soltanto" and "solamente" are both adverbs and conjuctions and their use is interchangeable. "Solo" compared to "soltanto" and "solamente" is more familiar.
*As an adverb their meaning would be equivalent to the English adverbs "only", "just". For example: Parlo solo/soltanto/solamente inglese (I speak only English); Ho speso solo/soltanto /solamente quattro euro (I spent just four euros).
*As a conjuction their meaning would be equivalent to the English conjuction "only" (but, however). For example: La pizza è buona, solo/soltanto/solamente un po' salata (The pizza is good, only a little salty).
"Solo" in addition is also an adjective and a noum (in Italian it is called "aggettivo sostantivato").
*As an adjectiveit means something similar to "alone"( as in "without company", "by oneself") or "only", "single". For example: Viaggio sempre solo (I travel always alone); Lavora da sola (she works alone); C'è una sola possibilità di riuscita (there is only one possibility for success).
*As a noum (male) it means sole, single and its meaning is similar to that of the adjective "only" (as in "the one"; "the one person"; "the only person"; "the only one"- in English "only" does not have a noum function) For example: Credo che tu non sia il solo ad avere questi dubbi (I think that you are not the only one to have these doubts). :-)