Fortress Def 3 for a +Def -Spd Genny?

There's basically no point in doing that. It doesn't save her from getting two-shotted by most of the common enemy threats (only takes 45 Atk or 47 *with* an HP Seal). You're going to want to give it on someone who'll not only make use of it, but basically make their strengths even bigger (Lukas is almost the perfect example). Not to mention, you're killing the only real good thing she has going for her, which is her good Atk. If you want to make the most out of her, look into her utility in healing (Rehabilitate, Live to Serve) as well as possible uses in Balms.