Which Chansey ?

I'm assuming your IVs are sta / atk / def. If so, stamina IV has very little impact on blissey because it already has insanely high stamina, an extra 3 HP does not make much of a difference when blisseys already have 400 HP, but they have low attack, hence why the difference between 15 and 14 attack IV is a bigger deal than on other pokemon.

3 hp probably means they get 1 more quick move before fainting, which don't do much damage because of their low attack, while boosting their attack increases their charge move damage by a small amount, which does add up because blisseys have high hp which means they are able to gain lots of energy through taking damage and therefore use their charge move several times per fight.

All 3! Then stack 'em up and have some impenetrable gyms. Ya only need to walk 200 kilos or so.

Number 1 is the best.

go for the one with the biggest att stat, followed by the defense stat and don't think about the stamina. The final % of the iv is just a cute number, you must always give pokemon with the better attack iv, followed for his worse stat between def and sta, unless the % difference is over 10%, always check on the CP calculator. CP calculation prioritize att base stat, and the IV add more points to the mon base stats, this way increasing the max CP (I think).