hi Misua,
from the info you have provided it is really difficult to give you an answer because you missed to give computational requirements and bandwidth ... type of VPN (MPLS or IPSec or else? ) and so on..
However, for 20 employee and three sites i would say that a 1941 would fit your needs with no excessive expense, providing a god scalability. then again , more info to suggest the right model would be a good idea. What model is the old router? Can you design topology?
Alessio