Try:

=IF(COUNTIF(ups!$A:A,main!$A3), "UPS", IF(COUNTIF(TFS!$A:A,main!$A3), "TFS",
IF(COUNTIF(OT!$A:A,main!$A3), "OT", IF(COUNTIF(polarspeed!$A:A,main!$A3), "POLARSPEED",""))))