Elin

this is what I think it means the second stage attaches to a single decoupler so that stage 3 can be symmetrical (without blocking the engines in stage 2.) stage 3 also connects to the same decoupler (or to the side of the stage 2 tanks) replace the pod with your 1.4t payload

this is the current suggested design X2 of stage 2 asparagused into stage 1, all three engines fire at the same time i believe you need to check the box "Use engine's atmospheric stats" also, from this site, http://wiki.kerbalspaceprogram.com/w/images/7/73/KerbinDeltaVMap.png i think your delta V to tylo is wrong