I took one of Master Yang Jun's hand form seminars about a year and a half ago. I had gone through the entire long form in class before I went, and I was very glad I did.
However, I also took the sword form seminar right after that, and had never done Yang style sword form before.
At the end of the seminar, I cannot say I knew the sword form. But I did know a good deal of it.
I don't think it's possible to learn any form in just three days time, but you can still learn quite a lot about a form in that amount of time.
I still practice the sword form to this day. I spent a good deal of time practicing the first part of the form, which I remembered clearly after the seminar, and then I spent a lot of time working on the rest of the forms from a DVD of Grand Master Yang Zhen Duo and Master Yang Jun. From that and my seminar notes I put together the rest of the form fairly well.
I guess what I'm trying to say is that if you have not taken the hand form before, you will still learn a great deal from taking the seminar.
At least, that's my opinion.
At the seminar I attended there were quite a few people there who had learned different styles of TCC who were learning the Yang form for the first time and they seemed to all do very well there.
Maybe others will have better, and certainly more informed, opinions than mine. It would be best to get more opinions on this before you decide, I think.