How to securely combine multiple sources of entropy?
If your keys, taken together, have sufficient entropy to support your desired security level, then HKDF (paper) is a conservative solution here, because it assumes that the input keying material to the HKDF-extract might not be uniformly distributed. 

The most conservative way of using it in your case would be to supply the concatenation of your keys as the IKM ("input keying material") parameter to HKDF-extract, and do not use the optional salt.  Then either use the PRK result directly, or as a further input to the HKDF-expand function.

A less conservative way of using it, if you are confident your inputs are indistinguishable from uniform random and mutually independent, would be to skip the HKDF-extract function supply the concatenated input directly to HKDF-expand.