Hi, I'm Bernard, a fellow Windows user. I'm happy to help!

For a 3-month game pass, it should show up in the account, and there is no need to redeem the code. Kindly check your subscription history if it's successfully activated: https://account.microsoft.com/billing/orders?ref=xboxsupport

If it doesn't push through, there might be an issue with the Microsoft Store. I recommend opening the Xbox app and redeeming it from there.

If the issue persists, I suggest resetting the Microsoft Store app and Xbox app by following the steps below:

Press the Windows key + R, then type in:

wsreset

Then hit OK

Method 2. Repair Windows Store and Built-in apps:

Press the Windows key + X, then select Windows PowerShell (Admin) or Terminal (Admin)

Copy, paste, and enter the command below:

Get-AppXPackage | Foreach {Add-AppxPackage -DisableDevelopmentMode -Register "_.InstallLocation)\AppXManifest.xml"}

Then hit Enter. Once the process is done, restart the PC and check.

For further assistance, you may check on this link: https://support.xbox.com/en-US/help/subscriptions-billing/redeem-codes-gifting/digital-code-solution