Can a normal sim card be used in a mi-fi device/modem?

The SIM card itself doesn't know what type of plan you have. That information is all held in the subscriber backend server.

If your SIM card supports data tethering, and your MiFi is not locked, and you have the correct APN settings. You should be good to go.

Most MiFi device use full-size SIM. And iPad uses the Micro SIM. You can get a SIM Adapter from eBay and then you can use your iPad SIM in your MiFi.

You have to get a mifi hotspot plan from your provider. Go to your provider's site to see what they require for a mifi plan.

It still won't work unless you have a hotspot mifi plan from "three".

Sorry - I meant to say: "If your account is tethering enabled" then the SIM card should work in the MiFi modem.