Just to be clearer, each
I'd make you squeak will contribute its effect, each with a limit of +3.
Therefore, in your first example, 2
IMYS plus Frodo with 2 wounds (no other wounds on hobbits) would cause a shadow number increase of +4.
In your second example, 2
IMYS plus Frodo and Merry each have 3 wounds, the shadow number would be +6.