It's an arduino board, model UNO R3. I have 3 5cmx10cm heating pad power required 5V DC up to 1A and can withstand up to 12V DC. These 3 heating pad are wired up onto arduino board with red wires connected to pin 8 pin 9 and pin 10 on the digital side of the arduino and the black wire connected to GND (ground). Lastly I have a toggle on and off switch connected to the arduino with the red wire attached to digital pin 2 and the black wire to 5V. The arduino uno R3 withstand 20voltage and the 5V heating pads can withstand 12V. I would like my 20V external power to heat up all 3 heating pads evenly, how can I do this? Is it possible? Thank you!
Yes, of course it is possible. But if all you need to do it to turn these heat pads on and off, an Arduino is perhaps not the most suitable technology.
But presuming these heat pads are a component of some more complex system that responds to other inputs or outputs to/from the Arduino, you will need a few more components. You need to accomplish two things that the UNO R3 cannot accomplish by itself, because your heat pads are very much too large a load for your UNO to power directly. (The power for a single LED bulb is about the most that you can draw from an output pin.)
You need to reduce your 20-V external power source down to something less than 12 volts to supply your heating pads.
You need to switch on and off up to 1 amp of current to flow to each of the three heating pads. The UNO cannot do this directly.
There are a lot of ways to reduce your 20-volt supply down to less than 12 volts. Some are much more efficient than others. Probably the lowest cost efficient way is to use your 20-V source as the input a small DC to DC voltage down stepping converter. These are available online from Chinese vendors for very low cost. Just search for "DC-DC converter down". A quick search finds one rated at 20 amps, with adjustable voltage output can be purchased for less than $10. These are pretty efficient devices that don't waste the power that a simple voltage dropping resistor would waste.
Then you need either one or three 5-volt relays of some type to switch the power on to the heat pads. The UNO pins output of 5-volts would not provide current to heat pads, put only enough current to energize the relay(s). The relay(s) can be either solid state, or coil energized. Just be sure the relay "contacts" are rated to handle well more than 1 amp if there are three of them, and more than 3-amps if only one. An Arduino relay shield with four relays, each rated for 10 amps, can be purchased for less than $5.
If you only every want all three heat pads energized at the same time, you only need one relay, and one Arduino output to energize it.
The very simplest solution, if you only want all the pads heating simultaneously, would be to wire the three heat pads in series. Then you could apply your 20-volts directly across all three heat pads, and you would not need any device to reduce your supply voltage. Each heat pad would have 1/3 of 20 volts across it, which is well under their 12 volt tolerance. And you would only need one relay.
Can you state the full goal of the project? Are you supposed to be turning the heating pad on and off with the Arduino? (The rest of my answer assumes that that is what you want to do.)
You will need a source of DC power to drive the Arduino. (You can't power an Arduino with AC.)
You will need a 5V relay so you can turn off the power to the heating pad from a logic pin on the Arduino. There are various relay modules made to be controlled by microcontrollers like Arduinos. This one, for example: https://www.jameco.com/z/VMA406-Velleman-Arduino-Compatible-5V-Relay-Module_2255306.html?CID=GOOG&gclid=EAIaIQobChMIr9bSioLT4AIVkpOzCh3X7gqbEAQYBSABEgLf1PD_BwE
Then you will be able to rig the Arduino to turn the relay on and off. You'll wire the power for the heating pad through the relay. You will also need a way to lower the voltage of your power supply to the 5V - 12V range you mentioned. A 1:2 step-down transformer would be a good choice for that.