Interesting find! It seems the private keys for these addresses are likely generated using a deterministic formula based on a sequence number. The increasing decimal values you've observed (3, 7, 8, 21...) suggest a simple mathematical progression.

Here's what I can analyze:

The pattern seems to be an increasing sequence, possibly exponential.

The provided examples show a clear relationship between the address number and the private key value.

Unfortunately, without knowing the exact formula or a starting point, it's difficult to predict the private keys for addresses 15 and beyond.