Notwithstanding any other provision of law, Executive order or regulation, any Presidential appointee or elected officer who leaves his position to enter, or who within ninety days after the termination of his position enters, the employ of the Agency, shall be entitled to the coverage and benefits of subchapter
III of chapter
83 of title
5, and chapter
87 of title
5, but not beyond the earlier of either the termination of his employment with the Agency or the expiration of three years from the date he entered employment with the Agency: Provided,