Orbi 970 router work as a satelite

No. but you can sell one and use the proceeds to get another satellite. 

Or keep it as a backup because you have so much invested in satellites. 

The Orbi 970 model’s are only Satellites (I.e. Access Points). The Orbi model 971 is the actual Router. It can be configured in either Router or Access Point mode (like the Orbi 970 AP only models.)

So, what is you question? Do you have two Orbi 970 satellites you are not using? Or do you actually have two Orbi 971 Routers and are you asking if the 971 routers can be reconfigured into Access Point mode (and work like the Orbi 970 Satellites)? Do you want to use all three Orbi’s (two model 970’s and one Model 971’s) behind another Router with all three of the routers in Access Point mode?