There are TWO separate things: (1) Material Participation, and (2) a Real Estate Professional.
If you do NOT make an election to "group" the rentals, the Material Participation is EACH property. If you make an election to "group" the properties, the Material Participation is based on all properties in the "group". If you have unallowed prior losses, I would NOT recommend making an election to "group" properties unless you sit down with a tax professional.
Once you figure out if you Materially Participated in the properties, then you determine if you have 750 hours COMBINED in all of the ones that you DID Materially Participate.
https://www.irs.gov/publications/p925#en_US_2017_publink1000104592
