For a set with $n$ elements, how many subsets are there whose cardinality is respectively $\equiv 0$ (mod $3$), $\equiv 1$ (mod $3$),  $ \equiv 2$ (mod $3$)? In other words, calculate
$$s_{i,n}= \sum_{k\equiv i \;(\text{mod} \;3)} \binom{n}{k}$$
for $i=0,1,2$. Your result should be strong enough to permit direct evaluation of the numbers $s_{i,n}$ and to show clearly the relationship of  $s_{0,n}, s_{1,n}$ and $s_{2,n}$ to each other for all positive integers $n$. In particular, show the relationships among these three sums for $n = 1000$.
1. **Introduction of the problem**: We need to find the number of subsets of a set with \( n \) elements whose cardinality is congruent to \( 0 \) (mod \( 3 \)), \( 1 \) (mod \( 3 \)), and \( 2 \) (mod \( 3 \)). We denote these numbers as \( s_{0,n} \), \( s_{1,n} \), and \( s_{2,n} \) respectively. 

2. **Using roots of unity**: Let \( \omega = e^{\frac{2\pi i}{3}} \), which is a primitive third root of unity. We know that \( \omega^3 = 1 \) and \( 1 + \omega + \omega^2 = 0 \).

3. **Generating functions**: The binomial theorem tells us that:
   \[
   2^n = \sum_{k=0}^n \binom{n}{k}
   \]
   Similarly, we can use the roots of unity to split this sum into parts where the exponents are congruent to \( 0 \), \( 1 \), and \( 2 \) modulo \( 3 \):
   \[
   (1+\omega)^n = \sum_{k=0}^n \binom{n}{k} \omega^k
   \]
   \[
   (1+\omega^2)^n = \sum_{k=0}^n \binom{n}{k} \omega^{2k}
   \]

4. **Summing the generating functions**: We can express the sums \( s_{0,n} \), \( s_{1,n} \), and \( s_{2,n} \) using these generating functions:
   \[
   s_{0,n} = \sum_{k \equiv 0 \pmod{3}} \binom{n}{k}
   \]
   \[
   s_{1,n} = \sum_{k \equiv 1 \pmod{3}} \binom{n}{k}
   \]
   \[
   s_{2,n} = \sum_{k \equiv 2 \pmod{3}} \binom{n}{k}
   \]

5. **Using the properties of roots of unity**: We can use the fact that \( 1 + \omega + \omega^2 = 0 \) to write:
   \[
   2^n = \sum_{k=0}^n \binom{n}{k}
   \]
   \[
   (1+\omega)^n = \sum_{k=0}^n \binom{n}{k} \omega^k
   \]
   \[
   (1+\omega^2)^n = \sum_{k=0}^n \binom{n}{k} \omega^{2k}
   \]

6. **Combining the sums**: We can combine these sums to isolate the terms we are interested in:
   \[
   s_{0,n} = \frac{2^n + (1+\omega)^n + (1+\omega^2)^n}{3}
   \]
   \[
   s_{1,n} = \frac{2^n + \omega (1+\omega)^n + \omega^2 (1+\omega^2)^n}{3}
   \]
   \[
   s_{2,n} = \frac{2^n + \omega^2 (1+\omega)^n + \omega (1+\omega^2)^n}{3}
   \]

7. **Simplifying the expressions**: We know that \( 1 + \omega = -\omega^2 \) and \( 1 + \omega^2 = -\omega \). Therefore:
   \[
   (1+\omega)^n = (-\omega^2)^n = (-1)^n \omega^{2n}
   \]
   \[
   (1+\omega^2)^n = (-\omega)^n = (-1)^n \omega^n
   \]

8. **Evaluating the sums for specific cases**: We need to consider the cases where \( n \equiv 0 \pmod{3} \), \( n \equiv 1 \pmod{3} \), and \( n \equiv 2 \pmod{3} \).

   - For \( n \equiv 0 \pmod{3} \):
     \[
     s_{0,n} = \frac{2^n + 2(-1)^n}{3}
     \]
     \[
     s_{1,n} = \frac{2^n - (-1)^n}{3}
     \]
     \[
     s_{2,n} = \frac{2^n - (-1)^n}{3}
     \]

   - For \( n \equiv 1 \pmod{3} \):
     \[
     s_{0,n} = \frac{2^n - (-1)^n}{3}
     \]
     \[
     s_{1,n} = \frac{2^n + 2(-1)^n}{3}
     \]
     \[
     s_{2,n} = \frac{2^n - (-1)^n}{3}
     \]

   - For \( n \equiv 2 \pmod{3} \):
     \[
     s_{0,n} = \frac{2^n - (-1)^n}{3}
     \]
     \[
     s_{1,n} = \frac{2^n - (-1)^n}{3}
     \]
     \[
     s_{2,n} = \frac{2^n + 2(-1)^n}{3}
     \]

9. **Evaluating for \( n = 1000 \)**: Since \( 1000 \equiv 1 \pmod{3} \):
   \[
   s_{0,1000} = \frac{2^{1000} - (-1)^{1000}}{3} = \frac{2^{1000} - 1}{3}
   \]
   \[
   s_{1,1000} = \frac{2^{1000} + 2(-1)^{1000}}{3} = \frac{2^{1000} + 2}{3}
   \]
   \[
   s_{2,1000} = \frac{2^{1000} - (-1)^{1000}}{3} = \frac{2^{1000} - 1}{3}
   \]

The final answer is \( s_{0,1000} = \boxed{\frac{2^{1000} - 1}{3}}, s_{1,1000} = \boxed{\frac{2^{1000} + 2}{3}}, s_{2,1000} = \boxed{\frac{2^{1000} - 1}{3}} \)