I am not sure of the legal precedent existing, but it is possible to read 3b as only requiring provision of the source to any third parties in posession of the written offer. In the long term it does not matter as anyone that has the written offer and has received the code may then redistribute, but it is still questionable if existance of a written offer somewhere is enough to be able to request thing by anyone. Say company A makes something for company B on the basis of GPL code. Company B gets the binaries and written offer of source. Company B uses the software internally, no further diatribution happens. Does an unrelated Company X have the right to ask Company A for the source? I do not think so. GPL was designed to protect rights of users of the software not just everyone. IANAL.