To determine the effective resistance between point A and C in the given circuit, we can use the concept of series and parallel resistors. Let's analyze the circuit step by step:

1. Identify the series resistors:
- The resistors R1 and R2 are connected in series as they share the same current. The total resistance of these two resistors can be calculated as R1 + R2.

2. Calculate the equivalent resistance of R1 and R2:
- The total resistance of R1 and R2 is R1 + R2.

3. Identify the parallel resistors:
- The resistors R3 and the equivalent resistance of R1 and R2 are connected in parallel as they share the same voltage. The total resistance of these two resistors can be calculated as:
1/(1/R3 + 1/(R1 + R2)).

4. Simplify the expression:
- We can simplify the expression further by multiplying the numerator and denominator by (R1 + R2) to get:
(R1 + R2)/(R3(R1 + R2) + R3).

5. Simplify the expression:
- Expanding the expression, we get:
(R1 + R2)/(R3R1 + R3R2 + R3(R1 + R2)).
- Simplifying further, we have:
(R1 + R2)/(R3R1 + R3R2 + R3R1 + R3R2).
- Combining like terms, we get:
(R1 + R2)/(2R3R1 + 2R3R2).

From the above expression, we can see that the effective resistance between point A and C is (R1 + R2)/(2R3R1 + 2R3R2).

Therefore, the correct answer is D) 3R.