I obtained an 3 year old Kenmore dishwasher. It looks like there is no
plug, and I have to use a 12 gauge wire to the 20A outlet to make the
connection - there is a small junction box in under the dishwasher.

Is this a correct way to make the connection?
Does the rule:
3'wire length4'
for the run from outlet to the dishwasher apply?

Thanks.

No, that rule does not apply.

The rule that does apply is called: What in the blue **** are you
talking about?