Three businessmen told the Senate Finance Committee today of Internal Revenue Service agents who, with guns drawn, broke down doors, terrified workers and forced teen-age girls to change clothes in front of male agents in raids at the men's homes and businesses that they said were unnecessary.

These aggressive paramilitary tactics were denounced as intolerable and ''fascist'' by Democratic and Republican senators as the panel heard testimony for a second day in its second round of hearings on I.R.S. abuses of taxpayers.

But the testimony by the three wealthy businessmen from Texas, Oklahoma and Virginia also showed that the agents had reason to suspect they had come across a $300 million tax fraud in one case and a drug operation in another.

Holes and contradictions in testimony by two of the businessmen prompted Senate Democrats to complain that the hearings were not a search for truth but a one-sided propaganda show by Republicans, whose polling data show that attacking the I.R.S. is their best strategy for raising money and winning votes.