While להגיד and לומר mean to say in the sense of making a statement, לספר means to tell, or tell over, a story or information (in Modern Hebrew).
For example:
היא מספרת לנו סיפור.
She’s telling us a story.
And:
הם סיפרו לכם מה הם עשו ביוון?
Did they tell you what they did in Greece?
This פיעל verb comes from the root ס.פ.ר, which also forms the word ספר – book. Likewise, a story is סיפור.
