I'd like to know if I can push 40 volts at 3 amps through an IRLZ44N MOSFET?
It can't handle 50 volts at 50 amps, that would fry it immediately. It can handle either 50 volts or 50 amps. 50 volts when turned off (Vgs &lt; Vth) and 50 amps (in pulses only, or with extremely good heatsinking!) when turned fully on (Vgs &gt;&gt; Vth).
If you're using it at 40 V, it should be perfectly fine. 40 V at 3 A is quite a bit of power, however, and you'll probably need liquid cooling for that much heat--that's 120 watts, more than your average CPU! Most likely, what you mean is 40 V when the FET is off (which is well within its rated 50 volts) and 3 A when it's on, which is (very!) well within its rated 50 A. If turned on hard enough, which for this FET is at a Vgs as low as just 4 volts, the power dissipation will be \$R_{ds,on}·I_D^2 = 315\ \mathrm{mW}\$, using the \$R_{ds,on}\$ at Vgs = 4 V found in this datasheet. This is low enough that you could probably get away with not using a heatsink at all, especially if it's pulsed.