If all you want is a 3A, 24V solid state switch, why are you using such a highly rated device? All of the IRL530 devices that I see say they can operate from 14A to 17A at room temp with 100V. Are you using this switch in some high temperature condition? are you going to configure it as a high or low side switch? Either way, that POWER MOSFET looks like it is going to need a MOSFET driver based on the size alone (depends on the turn on speed). Alas, you cannot exactly calculate the required current for turn on, but you can definitely estimate. Perhaps some documentation on POWER MOSFETs may help you, here is the wikipedia: http://en.wikipedia.org/wiki/Power_MOSFET and IRF (the company which makes your POWER MOSFET) provides a nice document http://www.google.com/url?sa=t&...BsLc-EwUYw

I believe on page 11 will show you the needed math to get your estimate. Plus you will need a flywheel diode (if it's not included with your MOSFET) because you're driving a solenoid.