1. Secondment is a temporary loaning at staff to a certain department, subsidiary etc to tackle a certain job. He will return after the tour of duty.
2. Transfer is the relocation of a staff from his present position or location to a different location, section, department or a subsidiary whether temporary or permanent.
3. Posting is more applicable to new staff, putting him to a place to start off.
That's how I understand, can be wrong.