To reproduce:
1. Open new project
2. Create 2 or more prefabs from any standard objects
3. In Project window, holding ctrl, select created objects
4. Check Inspector window's preview

Expected: each object is shown in their respective icon tile
Actual: each icon tile shows whole selected object group (check .gif attached)

Reproduced in: 2017.1.0p3, 2017.2.0b7, 2017.3.0a4
Not reproduced in: 5.5.4p2, 5.6.3f1, 2017.1.0b9, 2017.3.0a1, 2017.3.0a3
Regression starts in: 2017.3.0a4

Note: older regression had started in 2017.1.0b10 and was fixed in 2017.3.0a1
Note: another minor issue - until hovering on Inspector window's preview, one of previewed object is shown with darker lightning (seen in .gif)