What a sweet little trio! Sorry that you lost one.
Have you been able to weigh them yet? I would do so daily to make sure that they are all thriving until they have switched to adult food after 2-3 weeks old.
If the runt is under 60g, you should try to give him extra. Mushed hay based pellets are not bad. Some rescue give cubes of wholemeal bread, soaked in full fat goats milk. Under 50g, and Larry needs topping up with hand feeding to give him a chance. He will hopefully steady over the course of the day.
http://www.guinealynx.info/handfeeding.html#pups
Best of luck!