A.T. Robertson Commentary Acts 2:41

"They then that received his word were baptized: and there were added [unto them] in that day about three thousand souls." — Acts 2:41 (ASV)

They then (Hο μεν ουν). A common phrase in Acts either without antithesis as in 1:6; 5:41; 8:4,25; 9:31; 11:19; 16:5; or with it as here, 8:25; 13:4; 14:3; 17:17; 23:31; 25:4. Ουν connects with what precedes as the result of Peter's sermon while μεν points forward to what is to follow.

Were baptized (εβαπτισθησαν). First aorist passive indicative, constative aorist. Note that only those who had already received the word and were converted were baptized.

There were added (προσετεθησαν). First aorist passive indicative of προστιθημ, old verb to add, to join to. Luke means that the 3,000 were added to the 120 already enlisted. It is not stated they were all baptized by Peter or the twelve or all on the same day, though that is the natural implication of the language. The numerous pools in Jerusalem afforded ample opportunity for such wholesale baptizing and Hackett notes that the habit of orientals would place no obstacle in the way of the use of the public reservoirs. Furneaux warns us that all the 3,000 may not have been genuine converts and that many of them were pilgrims at the passover who returned home.

Souls (ψυχα). Persons as in verse 43.