These are some questions I was asked about Virtumart and the answers:
1. Can't move items from one category to another. Eg. I created 'Sale items'
by moving items from any of the categories into 'Sale items'. There was an option to do that in the older version. You just selected an item and then chose 'Move item'.
 There is no need to move items from one category to another as in VM3 you could assign item to several categories at once. please navigate to VM admin panel -> Products -> switch to tab "Product information" -> select reqiered categories at product categories select list (you coul choose more than one category). So if you want exclude some item from category simply press cross next to category name
category assignment
2. Don't know how to apply discounts as before, which I did each time for the sale items.
 Please navigate to VM admin panel ->Products -> switch tab to "Product information"
Go to Second block is called "Product pricing"
Check "override final" and print the desired value (see print screen for reference)
sale items 
3 What are Related Products?
"Related product" which gives you an opportunity to display products from one set at one in one page.
Please do following:
navigate VM3 admin panel ->Products ->switch to "Custom fields" tab
go to "Related product" section
add related products at selected list (just enter first letters of the product name and VM would show you this items)
don't foget to press save button
adding related products