the buggy boards definitely fit on the zapp I have one on mine for the kids I look after. (I'm a childminder) In my opinion they are fairly good and have improved but they do make the pushchair alot more difficult to push. My feet don't catch on it very often as I think it has been raised higher than they used to be. The only slight problem with one on the zapp is that if the child leans backwards it tips up as it is very prone to doing! I do use it if I have to though and it sounds like its a good idea for what you want it for.
I have no idea about the Silver Cross 3D though, if you go onto the official buggy board website it tells you exactly which pushchairs they fit. I would be interested to know too cos thats the pram I am going to get for my new arrival later in the year!