I'd be glad to share with you some insights about QuickBooks Online (QBO) subscription.
You'll need to pay the subscription first before you can use your QBO account or avail of our free 30 days trial. Also, if you've avail of our 30 days free trial, you'll have to pay the original price when subscribing.
If you wish to avail of our discounted price for 3 months, you'll need to subscribe to a new QBO subscription. You can also visit our Pricing page to learn more about this.