If you mean actual 3D models, then you could recreate them from scratch in Daz3d (available at daz3d.com). It's free (although it's easy to then spend a lot on it for additional clothes, props, hair, poses) and relatively easy to learn (there are a few tutorials and a really good forum), and you'll get basic human 3D figures that are highly customisable. For the faces, there is software (sadly not free, but they have frequent sales) that will transfer 2D faces to a 3D character.
If you just want them to look like they're 3D, you can quickly use Nightcafe (you get five credits free) to create a picture using your 2D version as your start image, and provide a suitable text description (though, like me, you might find some words you want to use are censored). That will generate 4 pictures, based on your original picture, but the AI randomness can be a bit annoying. Worth a quick try though. I think you can install the underlying Stable Diffusion on your computer directly, if your computer's sufficiently high spec, so you can get round the censorship issues, but try it first just to see if it's what you want.
Just as an example, the Daz3d images above took me an hour or so each. For the Nightcafe AI one, I didn't even need a start-up image. I basically just typed "Alyson Hannigan in black underwear by Adam Hughes" and the AI generated it in a few seconds, so if you don't really need 3D-ness, just the illusion of 3D, then I'd go the Nightcafe route. NB Other AI generators are available, and there's a post about them somewhere on this forum.
Having said that, Daz3d does offer a lot more control, and if you want to use your characters time after time then you might want to go that route. Also it supports multiple characters a lot better (if your computer has the power). You could do the same thing in Poser, but I've never tried that.
Hope that helps. If not, I'd try googling, or maybe other forum users can provide suggestions.