Eden is the current default 3D mission editor for ArmA 3. It can be used for object placement which are imported to terrain builder.

Place objects normally. You'll find objects under Props icon in the right hand side panel.

Export objects by File → Export to terrain builder, then copy CTRL-C and paste CTRL-V that to a text file, then import this text file into terrain builder by file → import → objects.

See also Beginner Eden Editor Object Placement Tutorial and github [E2TB] Export to Terrain Builder