Tech support says "social media." We use minimal social media and haven't changed usage in years. What could cause the phone to go from using 1/4th of a gig a month to 3 gigs in three weeks? We have no teenagers in the family: the phone in question belongs to an adult.
What kind of phone is it? If it's an iPhone and recently got an update, then there is an issue with WiFi assist that results in higher than usual cellular data usage. Go to Settings --> Cellular --> WiFi assist. Check to see that this is set to OFF. When this is ON, if your cell data signal is stronger than the WiFi signal, your phone will disconnect from WiFi, and you will use your cellular data.
Another thing to check is Facebook, though this change happened some time ago. Are videos playing by default on the phone? Turn that feature off, so videos only play when you are on WiFi.
You can go into your settings and see which app(s) are using most of the data.
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