First of all I think you are messing a little with two concepts: band and carrier frequency.
Band: describes you how wide will your transmitted signal in the frequency domain. e.g.: audio band is from 20Hz to 20kHz so it is 20000-20=19980Hz wide.
Carrier frequency: is the frequency at which the signal is transmitted in the air. The wifi router you have at home has a carrier frequency of 2.4GHz, and the data it transmits has a band of 22MHz.
If you look at a wifi power spectral density you will then see a sort of rectangle, 22MHz wide, centered on 2.4GHz.
Now to your questions:
1) The carrier frequency must be the same, i.e. if I transmit my data over 1MHz you can't expect to receive it over 2MHz. Band is also usually the same, or wider in the receiver: if I transmit some data that is 100KHz wide you can receive it if your receiver can accept a band of 100KHz or more.
2) Absolutely yes. If I am showing my friend a picture on my phone can my girlfriend see it if she's looking at it too? Well she probably could see it anyway but you got the point.
3) Devices 2 and 3 can not talk to 1. Different carriers = different languages. In order to get all the three talking together they must share the same carrier, or each one should have its private carrier with the other two, e.g.:
- 1MHz between 1 and 2
- 2MHz between 2 and 3
- 3MHz between 3 and 1
Sometimes devices use different carriers for uplink and downlink, as you sensed. That's true for wifi: the carrier is at 2.4GHz, the band is some 22MHz wide, but this band is divided in two slices: one for uplink and one for downlink. Assuming the slices are equal you can see all this as two bands 11MHz wide, one centered on a 2.4GHz-5.5MHz carrier, the other centered on 2.4GHz+5.5MHz.