A 1k base resistor is ok for a darlington transistor (TIP120 etc.), but not for a common NPN transistor.
A normal BJT needs a base current of 5-10% of the collector current when used as a switch.
In this case 50-100mA, which an Arduino pin can't provide.
So use a TIP120 with 1k base resistor or a logic level mosfet to switch those three fans (~1Amp).
Leo..