Two inputs are connected already. How should you connect the third input so that you can use a 3 input AND gate as a 2 input one.
Connect the extra input to the power supply so it is hardwired to a logic 1. Connecting two inputs together increases the loading on the signal connected to those inputs, causing slower transitions. If possible, choose the input that connects to the gate of the NMOS transistor that is closest to ground as the spare input. Permanently tying that input high will cause the internal capacitance at the drain of this transistor to be held at a solid low level and should improve the rising transition of the AND gate output.