"Now the word of Jehovah was rare in those days; visions were not widespread" (v. 1b). This indicates that Samuel's prophesying in chapter three was at the time when the word of Jehovah was rare and visions were not widespread.
According to verse 2, Samuel's first prophesying was also at a time when the waning Eli's eyesight was so dim that he could not see.
Furthermore, Samuel began to prophesy at a time when the lamp of God had not yet gone out in the temple (v. 3). God's speaking had stopped, but there was still some light. Applying this to today's situation, we may say that in degraded Catholicism and Protestantism there is still a little enlightening. This is God's mercy.
At the time of chapter three, Samuel "did not yet know Jehovah, and the word of Jehovah had not yet been revealed to him" (v. 7). As described in verses 4 through 9, Samuel had a new experience: Jehovah called to him, and he began to hear Jehovah's speaking.
The first prophecy spoken by Samuel concerned God's severe judgment upon the house of Eli (vv. 10-18).