I am involved with a civic non-profit group that operates a bar and
restaurant. Not long ago, we bought and had a point-of-sale touchscreen
system installed. It has 6 touchscreen terminals. There are also 3
terminals which only access a part of the database for manangement and
perpetual inventory reports/data entry.

The system is like ones found in many big restaurant/bar chains around the
world.

All the terminals go to a central processor in another city.

The three terminals for management reports and inventory can only have one
terminal online. While someone is using one, the other two are "locked
out".

Is there some kind of sharing device we can install which will let the three
terminals share and operate simultaneously?

Don't know if I've included enough info to answer this or not, but hope so.
I really don't know any more about it than I've stated above. We are
seriously limited by having the single terminal access.