How much internal memory is available for APPs on a 32GB tablet It is my understanding that 8 GB on a TABLET has 748mb available for apps and the rest dedicated for movies, music, documents etc. Of course there will be the boot, system OS stuff. 
Optionally there can be preloaded system apps which will count against the 748mb.

So I assume 32 GB would have 748mb X 4 or almost 3GB for applications. Is this correct ?
This is not exactly correct.
You only need to subtract the space required for the system once.
If you are on Android 4.4 you should have everything minus the system available for any purpose as apps, data and private files share the same storage.
Otherwise I assume around half of the available space can be used for apps and the rest for private data.