Hi
abalogh1026She joined ...,
she was injured ... ,
and she was discharged ...
the three verbs are in the simple past tense, so grammatically perfect.
The only difference is the "she joined" is an active voice (which means that she did the action),
while "was injured" and "was discharged" are a passive voice (which means that someone did these actions upon her).
Using passive and active voice is not related to the tense, but related to the meaning.
the tense only reflects when the action is done (the time), while the active/passive voice reflects who did the action.
To clarify further, if it is written "she injured" and "she discharged", this would be grammatically correct (still past tense), but would be wrong because of its incorrect meaning (because she is not the one who injured or discharged someone else)