Preheat oven to 375 degrees F. Bring about 1 inch of water to a simmer in a large saucepan. Put 3/4 cup sugar, 3 egg whites, cream of tartar and 3 tablespoons water in a metal bowl that will fit over the saucepan. Set the bowl over the gently simmering water and beat with an electric mixer on low speed, moving the beaters around the bowl constantly, until an instant-read thermometer registers 140 degrees F, 3 to 5 minutes. Increase the mixer speed to high and continue beating over the heat for 3 1/2 minutes. Remove the bowl from the heat and beat until the meringue is cool, about 5 minutes. Beat in vanilla.