Hi guys! I've been reading some visual novels that use 3D models and graphics as visuals, I've looked at the credits but it doesn't say anything, does anyone know what they could have used? 

They could have used almost anything. Poser and Daz 3D are well-known apps for making characters specifically, but hardly the only options. Why not ask in each game's comments / community?

Edit: or, you know, at least link to (some of) those games so people who know more can take a look.