Topology
Topology Flow
When bridging between two meshes that have different numbers of edges / vertices, to conserve the quad based mesh and avoid n-gons use the knife tool K in a similar fashion:
This also redirects the topology flow in a sort of 180° U-turn.
The second smaller zone is made by insetting the two faces with B for boundaries activated.
Going from 3 to 1
This construction can bring 3 edges to a zone with one edge. Slide the middle edge inward, select the three edges and F to fill then the 1:1 edges and F to fill.
Notice the 180° turn on the edge flow again.
Connecting 3 planes
Keeping quads in the mesh enables a better control over the shape afterwards.