Final answer:
The "3R" rule in the context given refers to significant figures and electrical resistance in circuits, with rules specifying significant digits and calculation of equivalent resistance.
Step-by-step explanation:
The "3R" rule typically relates to various contexts, such as environmental sustainability (Reduce, Reuse, Recycle) and photography (Rule of Thirds). However, given the examples provided, this rule appears to be related to significant figures in measurements for the first part, and electrical resistance in simple circuits for the solution part. The first set of rules specifies how to determine which numbers in a measurement are significant. Rules state that all nonzero digits are considered significant (rule 1), and leading zeros are not significant (rule 3). The solution part speaks about how the equivalent resistance in a circuit changes when resistors are placed in parallel, where it suggests the equivalent resistance is one-third of the resistance of a single resistor (R/3).