"Players may be registered with a maximum of three clubs during one season. During this period, the player is only eligible to play official matches for two clubs. As an exception to this rule, a player moving between two clubs belonging to associations with overlapping seasons (i.e. start of the season in summer/autumn as opposed to winter/spring) may be eligible to play in official matches for a third club during the relevant season, provided he has fully complied with his contractual obligations towards his previous clubs. Equally, the provisions relating to the registration periods (article 6) as well as to the minimum length of a contract (article 18 paragraph 2) must be respected."
Taken from FIFA's Regulations on the Status and Transfer of Players.