Note 4 at Ac 2:41: Many people have argued that 3,000 people could not have been water baptized in one day, especially since there was no naturally flowing streams or pools within the city of Jerusalem. However, there were man-made pools within the city that would have proven adequate.

The pool of Siloam, immediately south of the temple, would have been adequate. It was about 60 feet long and averaged 16 feet wide. This was the pool that Jesus instructed the blind man to go to and wash in (Joh 9:1-7). The upper and the lower Gihon pools were also nearby and suitable for water baptisms. The upper Gihon pool was 316 feet long and 218 feet wide. Archaeologists have also uncovered many ritual immersion bath pools adjoining the temple complex. There were more than enough pools of water to perform 3,000 baptisms.

As to the time that it would take to baptize 3,000 people, that also could have been easily done. If only 1 person was baptized every minute, or 60 per hour, 12 men could have baptized 3,000 people in a little over four hours.

Also, as mentioned in note 6 at Mt 28:19, any believer can administer water baptism, so there could have been many more than twelve baptizing. It is also possible that the people baptized themselves without another's assistance, as the converts to Judaism did in the temple's ritual baths and as some people think John the Baptist administered water baptism ("Understanding the Difficult Words of Jesus" by Dr. Roy Blizzard).