I know that if I am multiplying/dividing my answer is the least number of significant figure in the data and if I am adding/subtracting my answer is the least number of decimal places i the data but what if I have a constant for example:
If I want to find the average of the data so I'm going to add them then divide by the number of data:
$$\frac{3.05 \cdot 2.05 \cdot 1.05}{3}= 2\text{ or }2.19$$
will the answer be one significant figure because of the three even if the three is not part of the data or will the answer be 3 significant figures as this is the least number if significant figures in the data being calculated.
Since the $3$ in this case is known to be exactly $3$, you can treat it as if it had as many significant digits after it as the rest of your data (or better).
Hence you can write it as:
$$\frac{3.05 \cdot 2.05 \cdot 1.05}{3.00} = 2.19$$