I need symbols for alpha, beta and gamma

You can borrow some from me: α, β, γ. Please send them back when you're done.

if you don't know how to type them, open the Glyphs windows from the menu bar/type/glyphs and look for them there. double clickling on the character will add it to your Type cursor.

edit: you do need a font that supports them. a lot of them do.

Are you talking about the Greek letters?