Complex sentence: Truro City were the only one of the three to win the FA Vase while in Division One, while none are current members of the Western League, as all three have since progressed to the Southern League.
Split into simpler sentences: Truro City were the only one of the three to win the FA Vase while in Division One. Neither of the former clubs are current members of the Western League, as both have since progressed to the Southern League.