As others say in the comments, the question makes no sense because it does not state the probability of what. The probability of the die falling to a surface is presumably one, for example, regardless of how loaded it is. I will assume it means the probability of rolling a six (I know this is by no means the only possibile interpretation of this ambiguous question).
The wording of the question asks for you to be "at least 84% sure that the sample probability will be within 3% from the actual probability." The "at least" is surely significant. You can answer the question by assuming the worst case scenario - the die is loaded so six shows up 50 percent of the time so the variance of the number of sixes is $n(\frac{1}{2})^2$. Solve from there using usual methods.
So my answer is no, you cannot assume p is 1/6. As you have been asked to be conservative ("at least") you must assume the worst case scenario, which is p=1/2.