Suppose $a$ and $b$ are positive integers with a curious property: $(a^3 - 3ab +\tfrac{1}{2})^n + (b^3 +\tfrac{1}{2})^n$ is an integer for at least $3$, but at most finitely many different choices of positive integers $n$. What is the least possible value of $a+b$? 
1. We start with the given expression \((a^3 - 3ab + \tfrac{1}{2})^n + (b^3 + \tfrac{1}{2})^n\) and note that it must be an integer for at least 3, but at most finitely many different choices of positive integers \(n\).

2. To simplify the problem, we multiply the entire expression by 2 to clear the fractions:
   \[
   (2a^3 - 6ab + 1)^n + (2b^3 + 1)^n
   \]
   This expression must be an integer for the same values of \(n\).

3. For the expression to be an integer, it must be divisible by \(2^n\). We analyze the expression modulo 4:
   \[
   (2a^3 - 6ab + 1)^n + (2b^3 + 1)^n \pmod{4}
   \]
   Since \(2a^3 - 6ab + 1\) and \(2b^3 + 1\) are both odd, their sum modulo 4 is:
   \[
   (2a^3 - 6ab + 1) + (2b^3 + 1) = 2(a^3 + b^3 - 3ab + 1)
   \]
   This must be divisible by \(2^n\) for at least 3 values of \(n\).

4. We need \(2(a^3 + b^3 - 3ab + 1)\) to be divisible by \(2^n\) for \(n \geq 3\). This implies:
   \[
   a^3 + b^3 - 3ab + 1 \equiv 0 \pmod{8}
   \]

5. We now factorize \(a^3 + b^3 - 3ab + 1\):
   \[
   a^3 + b^3 - 3ab + 1 = (a + b + 1)(a^2 + b^2 - ab - a - b + 1)
   \]
   We need this product to be divisible by 8.

6. We consider the parity of \(a\) and \(b\). Since \(a\) and \(b\) are positive integers, \(a + b + 1\) and \(a^2 + b^2 - ab - a - b + 1\) must have opposite parity for their product to be divisible by 8.

7. We test small values of \(a\) and \(b\) to find the smallest \(a + b\) such that \(a^3 + b^3 - 3ab + 1\) is divisible by 8:
   - For \(a = 1\) and \(b = 5\):
     \[
     a^3 + b^3 - 3ab + 1 = 1^3 + 5^3 - 3 \cdot 1 \cdot 5 + 1 = 1 + 125 - 15 + 1 = 112
     \]
     \[
     112 \div 8 = 14 \quad \text{(divisible by 8)}
     \]
   - Therefore, \(a = 1\) and \(b = 5\) is a valid pair.

8. The least possible value of \(a + b\) is:
   \[
   a + b = 1 + 5 = 6
   \]

The final answer is \(\boxed{6}\).