I have a Keyboard Model No A1243 EMC No 2171 and the Key '3' on both the main and keypad doesn't work?

The A1243 board is a sealed unit. Any attempt to take is apart to clean or repair it essentially destroys it.

If you cannot get the key working with a brush or canned air†, it is time to go shopping.

† — do not shake canned air of hold the can upside down while using. Either will cause the can to blow propellant, resulting in ice on the object being cleaned that quickly turns to water.

Using the appropriate method for your iMac model try resetting SMC :

https://www.macworld.com/article/224955/how-to-reset-a-macs-nvram-pram-and-smc.html

i am confused also. Dirt or grit in the keyboard might cause a key to fail . . . but when every key works except the "3" on both the main keyboard AND the numeric part to the right on an extended keyboard .. . then i suspect something else is happening.

Maybe?

Or are both of the 3s link somehow?