Can I, as a DOA holder, use the Form 8110-3 issued by a FAA approved test house as a finalised compliance document?
No, test data issued by a test house should not be considered a finalised compliance document.
The DOA holder is responsible to establish the compliance data for its projects; this includes compliance reports made for flammability. All documents used by the DOA holder to demonstrate compliance must be signed off by a CVE of the DOA holder. This CVE can be external (for instance located within the flammability lab), in which case, the person must be linked to the DOA, being FAA DER or not. And the relation with the external facilities must be established by the DOA holder under the provisions of 21.A.239(c).
If a FAA DER is signing under such scheme, he shall sign according to DOA procedure, and thus FAA Form 8110-3 cannot be used.