‘They then who received his word were baptised, and there were added to them in that day about three thousand souls.'
The result was the ‘adding' to them of around three thousand people. ‘Three thousand' indicates a goodly and complete number in great contrast with the one hundred and twenty. There is a multiplication of people being received by the Lord. There may also here be an intended contrast with the three thousand men who were slaughtered because of Israel's disobedience with the molten calf (Exodus 32:28), the idea being that as a result of this that deep sin of Israel that immediately followed the giving of the covenant is reversed (just as the other tongues in Acts 2:4 pointed to the reversal of Babel). The new Israel can go forward as though that slaughter had never been.
They received his word and were baptised, probably in the River Jordan. These were ‘added to' the band of disciples, to the one hundred and twenty. The figure should not actually surprise us. There must have been tens of thousands who had heard and responded to the earlier teaching of Jesus who would simply be waiting for the news to reach them of His resurrection. He had been exceedingly popular.
The suggestion that Peter's voice could not reach three thousand people need not detain us. Those with a stentorian voice have no difficulty in reaching such numbers under reasonable conditions, and tests in Jerusalem have confirmed this to be the case.