You are using the free version, right? Then you can't use families, which would be the easiest way...
The only other way of doing this is making your event an 'or' block, so you'll have:
Bullet collides with A
Bullet collides with B
Bullet collides with C
(to make an 'or' block, right click the event and select "Make 'Or' Block"
And you can make it a sub-event of a blank event, just so you can hide it and it doesn't look like a mess