John Gill Commentary 1 Samuel 1:25

"And they slew the bullock, and brought the child to Eli." — 1 Samuel 1:25 (ASV)

And they slew a bullock
One of the three Hannah brought, unless the singular is put for the plural, and so all three were slain, some for sacrifice, and some for food perhaps; or if only one was slain, it might be offered as a sacrifice previous to the presentation of Samuel; or else was made a present of to Eli, at the introduction of Samuel to him, as follows:

and brought the child to Eli :
to be under his care, to be instructed and trained up by him in the service of the tabernacle; from hence it appears that Elkanah the husband of Hannah came along with her at this time.