1. **State the problem:** We have a bag with only red and blue marbles. We draw three marbles and define the random variable $R$ as the number of red marbles drawn. 2. **Determine possible values of $R$:** Since we draw three marbles, the number of red marbles $R$ can be 0, 1, 2, or 3. 3. **List all possible outcomes:** - $R=0$: No red marbles drawn (all blue). - $R=1$: Exactly one red marble drawn. - $R=2$: Exactly two red marbles drawn. - $R=3$: All three marbles are red. 4. **Construct the probability distribution:** Without knowing the exact composition of the bag or probabilities, the possible values of $R$ are simply $\{0,1,2,3\}$. 5. **Probability histogram:** To create a histogram, draw bars at $R=0,1,2,3$ with heights equal to their probabilities (which depend on the bag's composition). **Final answer:** The possible values of the random variable $R$ are $$\boxed{0,1,2,3}$$ representing the number of red marbles drawn.