Am I Sure? No. Sorry. I have not found a link that suggests what happens after the 3-months. (Does it 'end' or are you committed to a further 12 months?)
I am not a lawyer, and have difficulty interpreting the jargon, and no personal experience with this situation.! CONTACT CUSTOMER CARE
My understanding is that- if you want to use BOTH Lr and LrC (with your 300Gb of Cloud images) then you are going to need to change your Plan to the 'Photography Plan (1Tb) at the US$19.99/m and then use the 3-month redemption to extend your subscription.
https://helpx.adobe.com/manage-account/using/change-plan.html
And- FAQs https://helpx.adobe.com/creative-cloud/faq/CCPp.html
From this link- https://helpx.adobe.com/download-install/kb/redemption-code-help.html
"If you redeem a different plan with the same Adobe ID:
- Your subscription runs parallel to your existing paid subscription. For example, consider a scenario where you have an existing subscription to the Creative Cloud Photography plan with 20 GB of storage. However, you have a redemption code for the Creative Cloud All Apps plan. In this case, if you redeem the redemption code, your subscription to Creative Cloud All Apps runs concurrently with the existing Creative Cloud Photography plan"
Regards. My System: Lightroom-Classic 13.2 Photoshop 25.5, ACR 16.2, Lightroom 7.2, Lr-iOS 9.0.1, Bridge 14.0.2, Windows-11.