When having fireworks in your offhand and loaded fireworks into a crossbow with the multishot enchantment you can fire three fireworks. However unlike when loaded arrows into a multishot crossbow only one of the fireworks will do damage instead of all three fireworks. Remember to have all the fireworks hit the same target.

To test:

Step 1: Aquire multishot crossbow 

Step 2: Load fireworks that can do damage into your offhand 

Step 3: Shoot mob that will not die from one firework (example: chicken)

Step 4: Remember to have all the fireworks hit the same target.