Here are your parts
Parts for Whirlpool WDT790SAYM0 Dishwasher - AppliancePartsPros.com

Yes it could be any of the three parts you list.

The connecting harness can be checked with a meter.

Unfortunately I cannot find a pinout diagram for the keypad so trouble shooting it will be a pain.
I can tell you that many keys often share a common. So if this common wire goes bad then a number of keys do not work.
Also some keypads use steering diodes so you have to check the keys using both polarities of the meter.

There is also the possibility that the control board itself has a problem.

So I would do the below cleaning of the plugs and receptacles between the keypad and the control board.
Check the harness between the two parts with a meter to be sure it is OK.
If you have patience and some electronic experience you could check the keypad keys with a meter but this may tbe a pain. There is a possibility that you could trace the key tracks on the keyboard to figure out which pin out is which key.

The most likely culprit is the keypad itself.

Connector Cleaning / Checking
Unplug the unit
Open it up
Remove the ribbon cable from the keypad to the control board at the control board.
Clean the edge connector. Use electronics contact cleaner or isopropyl alcohol.
If it is the type where there is not a connector just the insulation is removed, it can be cleaned with a pencil eraser. Do not use an ink eraser as it is too abrasive.
Check the tracks in the ribbon cable for hairline cracks. A magnifying glass helps here.
Would not hurt to clean the connector on the board. Use electronics contact cleaner or isopropyl alcohol and a tooth brush. Be gentle and do not reconnect till all the solvent evaporates.
Reconnect everything and give it a try.

It is a volt/ohm meter also known as a multimeter.
It measures ohms, AC volts, DC volts and low amperage current.
It has a digital readout.

Yes they are available at local hardware stores etc.

Below is my standard blurb on meters.

If you do not own a meter, I would suggest you purchase a one. You can get a decent digital multimeter for under $20.00. You do not need fancy though it is nice if the leads are a couple feet long.
If it saves ordering one unnecessary part it has paid for itself and you end up owning a useful tool.
Most places will not let you return electrical parts so if you order it, you own it.
A couple things to watch when measuring ohms and continuity
1. Always remove power from the machine otherwise you could blow your meter.
2. Always disconnect at least one side of any device you are checking. This eliminates the possibility of measuring an alternate/parallel circuit path.
3. When checking for closed contacts and continuity use the lowest scale (Usually 200 ohms). Then try higher scales. This scale is 0 to 200 ohms so if the device you are measuring is 300 ohms this scale would show an open circuit which it is not, you are just measuring outside the scale's dynamic range.
4. When you start always short the meter leads together. This will tell you that the meter is working and if there is any 0 offset.

There is a good STICKY at the start of this forum about it's use.