Answer
The three articles form the Civil Code's framework on human relations. Article 19 lays down the general standard — act with justice, give everyone his due, and observe honesty and good faith. Article 20 gives a right to damages against anyone who wilfully or negligently causes damage to another contrary to law. Article 21 gives a right to damages against anyone who wilfully causes loss or injury to another in a manner contrary to morals, good customs, or public policy — even if the act violates no specific statute.
In short: Article 19 is the standard, Article 20 covers acts that break a law, and Article 21 covers acts that are technically legal but done in a way that offends morals or good customs (classic examples are a breach of promise to marry attended by seduction, or malicious prosecution). Articles 20 and 21 are the damages hooks through which an Article 19 abuse of rights is usually redressed.