setting orbi rbr760 as an main router and other rbr760 as an access point

Was it not possible to connect Room 3 satellite to the original router with Ethernet cable?  That would be the first choice.

For the system in Access Point (AP) mode, the SSID is irrelevant, except for one specific situation (see below ***).

Using the same SSID/password as the main WiFi system can be more convenient.  Conversation:

• "What's the WiFi password, dear?" 
• "Where are you in the house and which WiFi are you connecting to?"

Yuck. One set of credentials makes life easier.

However, the AP system is a separate WiFi network.  Devices will not roam seamlessly between the original system and this AP system.  This affects only devices that move (phones, tablets, laptops)  No matte what SSID the AP system has, a device connected to it will remain stubbornly connected as it moves around  the house until the WiFi signal gets so bad that it drops the connection and begins to look for another WiFi system that it recognizes.  In this case the conversation is:

• "Why is the WiFi so slow?"
• "Did you remember to turn WiFi off and back on again?"

Seamless roaming is the reason mesh WiFi systems exist and users find them superior to the previous WiFi expansion method of "WiFi Extenders".

*** There is one specific case where a separate SSID for the AP system makes sense: When a device that does not move appears to pick the wrong WiFi access point.  This complaint appears on the forum all the time. 'Why does my ???? connect to the router instead of to a satellite that is closer?  Isn't there a way to force a device to connect to a specific unit?"  If the AP system has a different SSID, then devices will connect only to it and not to the main system (with the original SSID).  If you have this specific problem, then a separate SSID makes sense.

Either way, not being able to keep one mesh system sucks.