added solution for day3 in python

[kushal7201]

Issue: #3

🔍 Checklist:

Please ensure the following before submitting your PR:

• [x] I have placed my solution in the correct folder (solutions/dayXX/).
• [x] My code follows clean coding practices and is efficient.
• [x] I have added comments in my code to explain the approach.
• [x] I have tested the solution on LeetCode and it works correctly.

🔧 Solution Details:

• Language used: Python
• Time Complexity: O(n)

🌟 Additional Notes:

• Calculate Total Sum: Find the sum of all elements and compute its remainder when divided by p.
• Check Divisibility: If the remainder is 0, return 0 (no need to remove any subarray).
• Prefix Mod Dictionary: Track prefix sums modulo p using a dictionary to find subarrays that can make the total sum divisible by p.
• Iterate Through Array: For each element, update the prefix sum and check if a valid subarray exists using the dictionary.
• Update Minimum Length: If a valid subarray is found, update the minimum length.
• Return Result: Return the length of the smallest valid subarray, or -1 if no such subarray exists.

[kushal7201]

@arya2004 Kindly review and merge my PR