I think zoom uses somewhere between 1gb-2.5gb an hour. It depends on how many is on the call, video quality etc. You might want to disable HD if you are limited with data.
Yes 3g data ia not going to work well. You might be okay on the lowest settings but if this is for work you may have a lot of disruptions. Or it might shrink the screen to manage on lower bandwidth? I've never used zoom so not sure how it handles low bandwidth exactly. It should work fine for audio only.
Your 1 to 2.5 gig per hour usage seems higher then the 1.5 meg per second speed conversion.
Now finding a conversion calc on internet
1.5 meg per second equals .68 gig
2 gig of data equals almost 3 hours of zoom
6 gig of data equals 8.8 hours of zoom
10 gig of data from your hotspot you suggested is just 14 hours
There is no way of knowing if a zoom call would be an hour or all day.
You would think cable company's. Know people working from home during covid pandemic
Can a non host of a zoom call change the HD settings?