Ok. This is where things unfortunately are going to get rather complicated. Looking in to it a bit more, it appears that Coinbase don't just use the WIFs they give as normal WIFs like any other wallet does to generate a single private key. Instead, they use the WIFs to generate a seed to then generate a master private key. This is a very weird way of doing things and I'm not aware of any other wallet or service which does this.

You have two option. The first is more secure but much more complicated. It will involve your airgapped computer, installing Linux, installing various Python tools, using those tools to decrypt your BIP38 encrypted key, and then following the instructions here (with a little modification) to generate your two xprvs, before using those two xprvs and your other xpub to recreate your vault in a wallet such as Electrum.

The second option is far easier, but not as secure. There is a fork of Coinbase's vault tool here (https://github.com/dlajarretie/multisig-tool) which replaces the now defunct BitPay API with BlockCypher's API which is still working just fine. However, I've never used this myself so cannot vouch for it. I've skimmed the code and it looks fine, but you will still be importing all your private keys on to an online computer with internet access, which is always a risk.

Great. I think this should be relative straightforward then, although since I've never used Coinbase vaults myself I can't confirm for certain.

Your key which starts with "L" and is 52 characters long is a compressed private key in the wallet import format (WIF).

Your encrypted shared seed which begins with 6P is an encrypted private key which uses the BIP38 standard. All BIP38 encrypted keys start with 6P. When you decrypt this with your password, it should (hopefully) provide you with a second key which mirrors your other one - starting with "L" and 52 characters long.

The combination of these two private keys alongside the third Coinbase public key should be enough to recover your wallet. We might hit a stumbling block regarding individual addresses or derivation paths, but we can cross that bridge when we get to it.

Next question: What format is the Coinbase public key in? Does it start with something like "xpub" or does it start with "02", "03", or "04"?

For the next steps, you are going to need to start importing these keys in to a wallet or other software. Doing this on a computer with internet access brings risk of your keys being compromised. Do you have a spare computer or laptop which you can disconnect and do the next steps offline?

Have you by any chance tried contacting Coinbase exchange and BitPay support for help?
Coinbase probably removed all vaults since 2018 but I see they still have page about that multisig vault available on their website, so they could help you to overcome unavailable Bitpay API.
https://www.coinbase.com/blog/multisig-vaults-on-coinbase

You can also download files and tried doing everything locally instead web tool, just follow this instructions:
https://github.com/coinbase/multisig-tool

I remember trying to help the user in that quoted thread at the time. If I remember correctly, he told me in another thread that his encrypted shared seed began with the characters "6P". Is this the case for you? Two additional questions regarding your user seed. Does it start with "5", "K" or "L", and is it 51 or 52 characters long?

If the answer to all those questions is yes, then it should be fairly easy to recover your vault in a wallet such as Electrum. If not, then we'll need to figure out exactly what format your two seeds are taking.