Explore Composite Numbers: Fill in the Blanks with 3

To solve this problem, we need to determine which number of the form $\square3$ is composite. A composite number has more than two distinct positive divisors.

• Step 1: Possible numbers from the format $\square3$ are 13, 23, 33, and 43.
• Step 2: Check divisibility and identify which numbers are not prime:
• $13$: Prime number as its only divisors are 1 and 13.
• $23$: Prime number as its only divisors are 1 and 23.
• $33$: Composite, divisible by 1, 3, 11, and 33.
• $43$: Prime number as its only divisors are 1 and 43.

Step 3: From the analysis, 33 is a composite number.

Therefore, filling the blank with the digit 3 gives the composite number $33$.

Thus, the solution to the problem is to fill the blank with $3$ to form a composite number.