A booking status shows
2 available under the Sr. Citizen quota.
Would a booking for two Senior Citizens and one Adult, show confirmation against 2 Sr. citizens and waitlist against 1 adult, if booked under a single PNR?
Short answer: NO
Long answer: The other answer is factually WRONG, I am sorry but you cannot combine senior citizen (or for most part) any quota with another ticket, the funny thing about senior citizen quota is you cannot even avail it when two senior citizens travel together, the definition of this quota is something to mean senior citizens travelling without support, so the answer is actually NO for your specific quota, I am not 100% sure of other quotas but many other quota tickets cannot be combined with open reservation.
An official confirmation is in this link this clearly says
"When travelling alone" for multiple quotas
Yes(Partially), If you would get the ticket under single PNR, your ticket would be considered partial confirmed ticket instead of confirmed ticket till all tickets get confirmed.And yes, you can travel on the ticket as well,even if you have e-ticket since your name will appear in reservation chart.
Name of the passengers whose names are partly confirmed/partly waitlist or partly RAC/partly waitlist, their names shall appear in charts including the waitlist passengers.