Three brave Ephraims which IVs are the best

I would probably take +atl +res in general, but that could depend on the set you plan to run on him. Do you have DC fodder? Do you plan to merge him? DB4?

+Def -HP. His res is shaky and lowering it is risky.

I'd take +def -hp because all of his stats are usable in someway, his speed/res is mediocre but can be viable with armor buffs. Res is used to battle dragons and tank mages so it's a poor bane while defense is one of the reasons that he's good but hp is the most useless stat out of the three banes and doesn't really affect him in anyway.

+Def/-HP is your best one to go with.