If $a, b, c$ are real numbers such that $a+b+c=6$ and $ab+bc+ca = 9$, find the sum of all possible values of the expression $\lfloor a \rfloor + \lfloor b \rfloor + \lfloor c \rfloor$.
1. Given the conditions \(a + b + c = 6\) and \(ab + bc + ca = 9\), we need to find the sum of all possible values of the expression \(\lfloor a \rfloor + \lfloor b \rfloor + \lfloor c \rfloor\).

2. First, we note that the floor function \(\lfloor x \rfloor\) gives the greatest integer less than or equal to \(x\). Therefore, we can write:
   \[
   \lfloor a \rfloor \le a, \quad \lfloor b \rfloor \le b, \quad \lfloor c \rfloor \le c
   \]
   Summing these inequalities, we get:
   \[
   \lfloor a \rfloor + \lfloor b \rfloor + \lfloor c \rfloor \le a + b + c = 6
   \]

3. Next, we consider the expression \((a - 1) + (b - 1) + (c - 1)\):
   \[
   (a - 1) + (b - 1) + (c - 1) = a + b + c - 3 = 6 - 3 = 3
   \]
   Since \(\lfloor a \rfloor \ge a - 1\), \(\lfloor b \rfloor \ge b - 1\), and \(\lfloor c \rfloor \ge c - 1\), we have:
   \[
   \lfloor a \rfloor + \lfloor b \rfloor + \lfloor c \rfloor \ge (a - 1) + (b - 1) + (c - 1) = 3
   \]

4. Combining the inequalities, we get:
   \[
   3 \le \lfloor a \rfloor + \lfloor b \rfloor + \lfloor c \rfloor \le 6
   \]

5. The possible integer values for \(\lfloor a \rfloor + \lfloor b \rfloor + \lfloor c \rfloor\) are 3, 4, 5, and 6. We need to check if each of these values is attainable.

6. To check if these values are attainable, we need to find specific values of \(a, b, c\) that satisfy the given conditions and result in the desired floor sum.

   - For \(\lfloor a \rfloor + \lfloor b \rfloor + \lfloor c \rfloor = 3\):
     - This is not possible because \(a, b, c\) are real numbers and their sum is 6. The minimum value of \(\lfloor a \rfloor + \lfloor b \rfloor + \lfloor c \rfloor\) is 4.

   - For \(\lfloor a \rfloor + \lfloor b \rfloor + \lfloor c \rfloor = 4\):
     - Let \(a = 1.5\), \(b = 1.5\), \(c = 3\). Then \(\lfloor a \rfloor = 1\), \(\lfloor b \rfloor = 1\), \(\lfloor c \rfloor = 3\), and \(1 + 1 + 3 = 5\). This does not work.

   - For \(\lfloor a \rfloor + \lfloor b \rfloor + \lfloor c \rfloor = 5\):
     - Let \(a = 2.5\), \(b = 2.5\), \(c = 1\). Then \(\lfloor a \rfloor = 2\), \(\lfloor b \rfloor = 2\), \(\lfloor c \rfloor = 1\), and \(2 + 2 + 1 = 5\). This works.

   - For \(\lfloor a \rfloor + \lfloor b \rfloor + \lfloor c \rfloor = 6\):
     - Let \(a = 2\), \(b = 2\), \(c = 2\). Then \(\lfloor a \rfloor = 2\), \(\lfloor b \rfloor = 2\), \(\lfloor c \rfloor = 2\), and \(2 + 2 + 2 = 6\). This works.

7. Therefore, the possible values for \(\lfloor a \rfloor + \lfloor b \rfloor + \lfloor c \rfloor\) are 4, 5, and 6. The sum of these values is:
   \[
   4 + 5 + 6 = 15
   \]

The final answer is \(\boxed{15}\)