1) a. Magnifying glass: you use this to detect ore deposits. I recommend you use this by pressing close to a "near" or "very close"
b. Compass: points in the general direction of ore deposits.
c. Dynamite: detonate to reveal ore deposits. Again I recommend using this by pressing close to a "near" or "very close"
To use any of the above you need to choose a pick or shovel then while mining press on the one you want to use (once) then tap on the area where you wish to use it.
2) I'm guessing that when you used the magnifying glass there was no ore in that area
that would make it seem as though it didn't work.
3) yes unfortunately we only get three free tries with each. Don't worry though as many players don't use any of these and find ore ok without them.
I would recommend saving your freebies for the time being anyway.... Perhaps for that tough train that needs ores or items made using ores. Or if you find yourself in a situation where you have only one hit left and your previous hit was a "near" or "very close"
Keep in mind you only get 12 hits with each tool (shovel or pick) it's worth keeping track of so you don't accidentally spend gems you have been saving.
Hope this is helpful
Happy mining
Happy farming