Added solution for 15. 3Sum Leetcode problem

LeetCode/Problems/Python/15_3Sum.py

@@ -0,0 +1,18 @@
+class Solution:
+    def threeSum(self, nums):
+        res = set()
+        targets_dict = {}
+
+        nums.sort()
+
+        for i, num in enumerate(nums):
+            targets_dict = {}
+            target = -num
+            if(i>0 and nums[i] == nums[i-1]):
+                continue
+            for num2 in nums[i+1:]:
+                if(num2 in targets_dict):
+                    res.add((num, num2, targets_dict[num2]))
+                targets_dict[target-num2] = num2 
+
+        return res