If 3D functionality is activated in a project, the button "Switch between 2D and 3D mode" is available, whereby the button displays the unselected view (i.e. the one which is switched to by clicking).

Use of 3D mode in WebOffice flex client
The 3D functionality is based on the ArcGIS API for JavaScript. For its availability, the following prerequisites must be fulfilled:
•Supported coordinate systems:
oWGS84 and Web Mercator Auxiliary Sphere are supported as coordinate systems
oCached services must be available in WGS84 or Web Mercator Auxiliary Spher
oDynamic services can exist in any coordinate system and are reprojected on the fly
•The following requirements apply to tiled services:
oThe tiles need a size of 256x256 or 512x512 pixels
oThe scale steps must differ by a factor of 2
oLevel 0 should not consist of more than 30 basic tiles
oAll tile layers must have the same tile scheme and the same coordinate system
Note: Further information can be found in the documentation of the ArcGIS API for JavaScript:
https://developers.arcgis.com/javascript/latest/api-reference/esri-views-SceneView.html
http://doc.arcgis.com/en/arcgis-online/create-maps/choose-global-local-scene.htm