If you're fermenting fruit, using a bucket is a huge help. You'll want to manage the cap (you can search for "cap management" for lots of details there), which involves pushing it down into the liquid frequently. A bucket with a large opening makes this a lot easier.
Also, you can look into the foam control drops. Medsen swears by them.
For your questions, yes you can do 2 gallon primary in a 3 gallon container. Heck, you can use a 100 gallon container if you want. (ok, that would be a little silly) Headspace is not a problem during vigorous fermentation, unless you don't have enough!
When to add fruit is the $64,000 question. It really depends on what you want. The earlier you add the fruit, the more fermented character the fruit flavor will have. Think wine vs. grapes. Also some of the aroma compounds can be blown off during vigorous fermentation, so if you've got a really delicate fruit aroma (peach, watermelon, etc) then you're going to either add so much that the aroma loss isn't a big deal or add the fruit later. Raspberries usually have a pretty potent aroma, so this isn't a large concern here. Late fruit additions will give you a more "fresh fruit" flavor.
So...you just need to decide where to go given all that information. Very often folks will add some fruit in primary and then add more afterward to boost the fruit flavor. Keep adding fruit until you're happy. You might not want to add any, or you might find yourself with a trunkload of berries.
On that note, you should probably get more berries.