PhotoMosaic: allow sets of three portraits.

models/PhotoMosaic.php

@@ -115,6 +115,10 @@ class PhotoMosaic
 		// Sort photos by priority and date captured.
 		usort($photos, 'self::orderPhotos');
 
+		// Three portraits?
+		if ($num_portrait === 3)
+			return [$photos, 'portraits'];
+
 		// At least one portrait?
 		if ($num_portrait >= 1)
 		{