I am thinking of making a custom keyboard for a VJing software. I couldn't find momentary switches of size greater than 15mm, so I am thinking of buying these MX keys (they have 3 pins.) I could'nt find a wiring diagram online. So will you be able to help me out with the wiring diagram (I will connect them to an Arduino Uno R3.)
You need something called a datasheet. That will have all (or most) of the information necessary for a knowledgeable designer to use the parts.
In this case, you know what the part is: a switch
You know the model series: MX
You should know the manufacturer: Cherry Americas LLC (owned by a German conglomerate and probably manufacturing the switches in Zuhai China, near Macau).
So google that and you should come up with this datasheet. A similar one is here Unfortunately I don't see any with an odd number of contact pins, unless you count the plastic locating pin in the center.
Anyway, all the information you need should be there for whatever type of switch you are looking to purchase. Unless you want the kind with a matrix diode or LED incorporated, in which case contact the manufacturer to find out the polarity of the diode(s).
Cherry Switches come in 3 pin and 5 pin varieties. In both cases there are only 2 electrical pins, as these are just momentary switches. The other pins are plastic and for mounting stability. They dictate whether they will be mounted on a PCB or not (5 pin is for PCB).
If you find a switch with more than 2 pins, you can use a multimeter in continuity mode to find the right pins. The pins you want are connected and disconnected when the switch is pressed and released.
Continuity mode on a multimeter is a mode where the meter beeps if the 2 probes are connected together.
Once you've got the switch figured out, search "how to connect switch to arduino" and you will find hundreds of tutorials to walk you through it. I won't get into that because it's already been said 1000 times.