维持 means 'to maintain, keep, or preserve' — it focuses on keeping an existing state, relationship, or condition going, often against challenges. It is more about sustaining something that might otherwise weaken (e.g., 维持友谊 'maintain a friendship'), while 保持 (bǎochí) is about continuing a desirable state without change (e.g., 保持安静 'keep quiet'). 维护 (wéihù) is stronger, meaning 'to safeguard or defend' rights, interests, or justice.