In fact they could have sent five. From the press release on the WPA website:... it's odd that some countries have a team of three and others a team of four. ...
In total, 16 teams from around the world will represent their country at the $300,000 event, which pays $120,000 to the winning team. Teams are comprised of a minimum two male and one female player. Teams can register up to five players (three male and two female) for the event, but only three players will take part in each round.