Rub poussin with 1 tablespoon of the olive oil. Lightly season with salt and pepper. Place 1 lemon wedge and 1 sprig rosemary in cavity of each bird. Arrange in a large, heavy roasting tin, and arrange garlic cloves around them. Roast in preheated oven for 25 minutes.
Reduce oven temperature to 180 C / Gas mark 4. In a mixing bowl, whisk together wine, chicken stock, and remaining 2 tablespoons of oil; pour over birds. Continue roasting about 25 minutes longer, or until birds are golden brown and juices run clear. Baste with pan juices every 10 minutes.
Transfer to a platter, pouring any cavity juices into the roasting tin. Tent the birds with aluminium foil to keep warm. Transfer pan juices and garlic cloves to a medium saucepan and boil until liquids reduce to a sauce consistency, about 6 minutes. Cut them in half lengthwise and arrange on plates. Spoon sauce and garlic around the birds. Garnish with rosemary sprigs, and serve.