As Fiat Lux mentioned, if you're going to add QBs to their desktop, you will need a 4th licenses.  

Another option might be to install one of the QB licenses on a small dedicated machine (e.g. Lenovo ThinkCentre M93P) and then have the two users remote into it when they need to access QBs.  You'd have to compare the costs of the computer, remote access software if you don't use Windows Remote Desktop, etc.  But it is worth considering.

And if you ever need additional people then you're all set.  This is no different than if the person gets out of the chair so the next person can sit down to work in QBs.  They are just "virtually" sitting down.