Use a 2D DP table where `dp[i][j]` represents whether the first `i+j` characters of `s3` can be formed by interleaving the first `i` characters of `s1` and the first `j` characters of `s2`.

Initialize the DP table with `dp[0][0] = true` (empty strings interleave to empty string) and fill the first row/column by checking if `s3` matches `s1` or `s2` when one string is exhausted.

For each cell `dp[i][j]`, set it to `true` if either: