3 hens, 1 laying, all same age

How old are they? That is enough nesting boxes for all of them. If they are still pretty young then it could just be that the other two have not started laying. If they are older is there any were else they would be wanting to lay them?

 

A few thoughts here -
How old are the birds and how long has the laying bird been laying? It sounds like you likely have three young birds and one has started to lay - which would simply indicate that the other two are not yet mature and ready to lay. Even with the same breed of bird individuals will mature at different rates.
Second - on the issue of the other box - do you know if your birds are sleeping in that box at night? That is the most common cause of the "litter box" appearance of a nesting box because of the amount of waste that is passed in the night hours. This is definitely undesirable and there are ways to correct the behavior.
Third - it is fairly easy to do a "stake out" and determine who is/isn't laying - just takes a couple hours of your time (or more, especially with larger flocks) on a given day and you don't have to be "right there" (if you have a nervous bird it is preferable not to be a disruption - but if your bird is calm and okay with your presence there is no reason you can't be a direct observer) - it is about knowing who is/isn't in the coop or run and who is going in/out of the nest box at the time the egg is left.....for flighty birds outside observation is best, for calm birds you can even go so far as to watch them actually lay the egg (if that's your cup o'tea)
And, lastly, the obvious question - are you certain that they are all pullets? It may be helpful to post photos of your birds - not only can we be sure they are all female, but we can also see if we see any "getting close/almost ready" to lay signs showing in your birds that would give you an idea of how much longer your wait might be