Hi,
in Qlikview security is applied on document level and field level/data level.
for Document level if that document is needed to be seen from server than NTFS/DMS level authorization is also required.
Data/field level security is implemented using section access.
where you can use OMIT field to omit the fields at all or if you want only some of the data from that field to be shown for a user than it needs to linked to that user name or id somewhow.
You can not secure just a table for a particular user but you can use OMIT in section access which omits a field and if you use OMIT for all the fields in 3rd table then 3rd table will not be loaded for that user. but if you have some common field then rather then using OMIT for that field you can use data level security so, you can hide the values in that field which come from table3.
To read about section access in detail go to this link/document which will be a good starting point for section access.
http://community.qlik.com/docs/DOC-1853
Hope this helps.
...
Ashutosh
