Can I sacrifice creatures that are attacking before they are blocked?
Yes, you can sacrifice one of you attackers after blockers but before combat damage.

116.1b A player may activate an activated ability any time he or she has priority.
116.3a The active player receives priority at the beginning of most steps and phases, after any turn-based actions (such as drawing a card during the draw step; see rule 703) have been dealt with and abilities that trigger at the beginning of that phase or step have been put on the stack. No player receives priority during the untap step. Players usually don’t get priority during the cleanup step (see rule 514.3).

At this moment you are the active player and will receive priority after the declare blockers step. Now that you have priority you may activate an activated ability, so you can activate your Goblin Bombardment and sacrifice one of your creatures to kill one of your oponents blockers, for example.