Google drive: 15GB free, $9.99/mo for 1TB
OneDrive: 6.99/mo for 1TB
I suppose you can find other big fishes' pricing by yourselves, and I would not go with free service which can disappear tomorrow.
3TB is lot of data. Just calculate how long it would take to upload that amount of data.
OTOH - 3TB external drive is $100. Rotating two of them (one at home, one kept into a bank vault / friends house provides for enough "peace of mind".