I am sorry to learn of this situation.
Unfortunately, based only on what you posted, I am not sure what I can tell you to do.
You called the police department and filed a report against the landlord's security company for doing their job (asking unaccompanied minors loitering outside the gate what they were doing there).
I'm not sure exactly what the landlord cited as the basis for their 3 day, but they can use a general violation of the lease terms, or a much more specific violation depending on the terms in your lease, to support this notice.
It may be possible to fight this matter on the merits of the specific notice (the 3 day is not supported by the lease clause they cite for example), but this will depend on the specifics of your matter.
You can try mediating this dispute with your landlord, contact your local bar association and ask for referrals to third party neutrals (attorneys, retired judges, or professional landlord/tenant mediators), that may be able to help you reach a "mutually agreeable resolution."
I also recommend contacting your local bar association and asking about legal aid to assist you with eviction defense. It is possible to fight an eviction on your own "pro per" but you stand a much better chance of winning such a claim if you are represented by counsel (especially when your defense is going to turn on very specific legal arguments and nuances regarding whether or not calling law enforcement about staff doing their job is a "material" lease violation).