1. Three bridge APs in a row sharing a channel. Canceling no, but contending yes.
2. Three mesh APs in a row sharing a channel. Same.
3. Three pair of bridge APs in a row using (presumably) three channels. No contending, but traffic is duplicated in transit segments.
4. Three pair of bridge APs in parallel (all "root" bridges on the top floor) using three channels and sharing an Ethernet. No contending, and less transit traffic.
Note: WLAN bridge APs and mesh APs are proprietary. There is no standard behavior. The terms "root bridge" and "non-root bridge" are proprietary in this context and have nothing to do with similar standard terms used with the IEEE 802.1D Spanning Tree Protocol (STP).
I hope this helps. Thanks. /criss