Yes you will use your phone data allowance if you connect your computer/tablet to the web via the phone hotspot. Be very careful that automatic updates are switched off on the mac/tablet or you will start downloading OS updates over your phone, ditto cloud sharing of things like photos. The tablet/mac see the phone as normal wifi so they can kick start all sorts of downloads.
FWIW I used a mobile wifi device from Three with my mac/tablet and it works really well for normal browsing etc. IME the iPhone works less well.
What @somuk says is correct, the pure link between phone/tablet/mac wont use data but lots of stuff can happen in background. If you want to stop this just switch mobile data off on the phone temporarily.