I've moved this thread to 'Personal Finance' since your question is about membership eligibility rather than a credit card app. 
The criteria for each of the three CU/banks you mention are all different.
For PenFed, if you're not military yourself or related to someone who is eligible you can gain membership by making a donation to either Voices of America's Troops or the National Military Family Association ($17 one-time for either)... check the PenFed site for more info.
With NFCU, you can gain membership if you have a family member who is or was in the military, and they are quite generous in that 'family member' includes grandparents. Also, with NFCU the qualifying relative does not have to have been a NFCU member themselves or still be living. NFCU also lists 'household member' as a way in (as in, you live with a member or someone who is eligible) but I would personally avoid this if at all possible, since they watch it closely and you'll need to have your documentation absolutely perfect since it will almost certainly be checked at some point, which is a pain in itself.
USAA is the most restrictive of the three; to be a member you will need to either be past or present military yourself or have a spouse or parent who is/was a member (due to their own service or that of their parent or spouse). Many people have been disappointed to find out that they aren't eligible because their military (or otherwise eligible) parent was never a member and is now deceased. Also, with USAA you can gain membership from a parent, but not a child or sibling. USAA membership criteria has changed several times over the years, so if you don't qualify now it never hurts to occasionally check with them (website or phone) to see if perhaps eligibility requirements have changed in your favor.
@UncleB wrote:
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USAA is the most restrictive of the three; to be a member you will need to either be past or present military yourself or have a spouse or parent who is/was a member (due to their own service or that of their parent or spouse). Many people have been disappointed to find out that they aren't eligible because their military (or otherwise eligible) parent was never a member and is now deceased. Also, with USAA you can gain membership from a parent, but not a child or sibling. USAA membership criteria has changed several times over the years, so if you don't qualify now it never hurts to occasionally check with them (website or phone) to see if perhaps eligibility requirements have changed in your favor.
Support the check in with USAA from time to time. My membership was denied and finally when the dust settled they approved "honorably discharged military". A member of the MyFico family shot me a note and informed me of the change. Got in back in 2010. Since then there were membership modifications that became very confusing to all. That has been stable now for a while. Good luck in your pursuits 