Can I find out, ahead of court date, if Plaintiff has "surprise" proof of "posting". Is there discovery allowed in this matter?

Proof of service by posting is like proof of service by mail - someone signs a document swearing that they did it. No one guarantees it didn't get blown away by the wind or someone took it or that you read it, but they can still swear that you were served. They may bring the process server to court with them to testify as to the service, and all you can really do is ask them what your door looks like, what they used to affix the notice to the door, etc., but even the best cross-examination will probably leave their testimony believable.

Challenging service may not make a lot of sense here anyway. What are you trying to do, buy time? By increasing the effort to evict you, if your lease has an attorney's fees clause, you're increasing the landlord's legal fees, which will just get added to your debt.

If you're trying to stay and you want to argue that you would have paid the rent within the 3 days had you gotten the notice, you could try settling with the landlord if you get current.

Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.