i agree that the correct application will come down to cost and size.

from what you are quoting above, i would go with a VB.Net to Access database. it is simple and cheap and you will be able to implement it very easily and quickly. i've written dozens of VB/Access apps and have rarely encounted locking problems. in "most" cases, it's rare that 2 PCs will be writing to the same records at the same time, especially if you only have 3 PCs in total.

then once it is going and workign nicely, it is simple to evaluate and check to see if you need to upgrade. if the tables start to grow too large (as stated above), then migrating from Access to SQL server is very simple.