PI Planning schema for unpredictable backlogs
Team is working on Program1, Program2 and Program 3. Program1 & 2 having predefined content and Program 3 is not having any predefined content(backlog may come at any time). In this scenario how to define PI planning schema.
It sounds as though Programs 1 and 2 have no emergent work and hence no need for agility to be demonstrated. If there really is any complexity behind them, the precocious definition of work will put the success of those initiatives at risk.
Program 3 involves emergent work and hence planning ought to be undertaken each Sprint. The Product Owner should be able to communicate a Product Goal, and enough work should be refined and ready to allow Sprint Planning to begin.
Agree with Ian, the product owner needs to communicate the product goal. This should be enable the team to create epics at a high level (should be prioritized by PO). Next the team can break down the epics to stories (add proper acceptance criteria and ensure each story delivers value). Then in the sprint planning let the team decide what should be pulled in. Utilize the sprint review and ensure the right stakeholders are available for the same to get the best possible feedback.
Another point you might have to check is the dependencies between prog 1,2 and 3. You mentioned PI planning which is essentially from SAfe so my guess is the 3 programs have some kind of dependencies or together contribute to your product.
Both @Ian and @Anand gave you great answers based upon the Scrum framework that is defined in the Scrum Guide (https://scrumguides.org/).
However, it sounds like you are asking about how to accomplish something in SAFe. SAFe uses a version of Scrum that they created called ScrumXP (https://v5.scaledagileframework.com/scrumxp/). PI Planning (https://v5.scaledagileframework.com/pi-planning/) is a fairly regimented process in SAFe. And a Program Increment (PI) is also defined (https://v5.scaledagileframework.com/program-increment/).
Looking at those resources I would say that Program 3 is not ready to be included in PI Planning since it cannot be clearly understood and defined. It should be deferred to another PI Planning event so that it can be better prepared. Have you considered Program 4? Is it ready?