You can downgrade TWL_FIRM on sysnand as well as the DS Cart Whitelist. However if the DS Firecard did not work on 3DS back when it was on 2.0 firmware (1.0 TWL_FIRM will no longer work on firmwares on 2.x or newer, so you can't downgrade to a TWL that old) then it will never work on the 3DS no matter what CFW you use and even if you had downgraded both TWL and DS Cart Whitelist to oldest possible versions.
You'll need to find the CIAs for the 2.x TWL_FIRM and 1.x version of DS Cart Whitelist (Unlike TWL, you can use 1.x versions of DS Cart Whitelist). You will also need an old version of DevMenu to install it. Use the one provided on Gateway's official site. Obviously you'll need to be in a CFW or Gateway mode for it to install. Newer versions of DevMenu won't override TWL with older versions. (That and you could also just uninstall it first using FBI. But that's risky if you don't know what you are doing as you could accidentally uninstall the wrong system title)
Also, I could be wrong, but n3DS uses a separate version of TWL_FIRM and you can't downgrade that past what was originally provided on 8.x and newer. I don't think TWL_FIRM has even seen an update since the launch of the n3DS so on that there is nothing to downgrade it too. You can still downgrade DS Cart Whitelist on n3DS, but that means some DS Flashcarts still won't work. (though DS Cart Whitelist downgrade is enough to get Gateway's blue card to work)