/usr/include/t3/ is in libt3widget-dev 0.6.2-1build1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/t3/widget/
/usr/include/t3/widget/t3widget/
/usr/include/t3/widget/t3widget/autocompleter.h
/usr/include/t3/widget/t3widget/clipboard.h
/usr/include/t3/widget/t3widget/colorscheme.h
/usr/include/t3/widget/t3widget/contentlist.h
/usr/include/t3/widget/t3widget/dialogs/
/usr/include/t3/widget/t3widget/dialogs/attributepickerdialog.h
/usr/include/t3/widget/t3widget/dialogs/dialog.h
/usr/include/t3/widget/t3widget/dialogs/dialogbase.h
/usr/include/t3/widget/t3widget/dialogs/filedialog.h
/usr/include/t3/widget/t3widget/dialogs/finddialog.h
/usr/include/t3/widget/t3widget/dialogs/gotodialog.h
/usr/include/t3/widget/t3widget/dialogs/inputselectiondialog.h
/usr/include/t3/widget/t3widget/dialogs/insertchardialog.h
/usr/include/t3/widget/t3widget/dialogs/mainwindow.h
/usr/include/t3/widget/t3widget/dialogs/menupanel.h
/usr/include/t3/widget/t3widget/dialogs/messagedialog.h
/usr/include/t3/widget/t3widget/dialogs/popup.h
/usr/include/t3/widget/t3widget/extclipboard.h
/usr/include/t3/widget/t3widget/findcontext.h
/usr/include/t3/widget/t3widget/interfaces.h
/usr/include/t3/widget/t3widget/internal.h
/usr/include/t3/widget/t3widget/key.h
/usr/include/t3/widget/t3widget/key_binding.h
/usr/include/t3/widget/t3widget/key_binding_decl.h
/usr/include/t3/widget/t3widget/key_binding_def.h
/usr/include/t3/widget/t3widget/keybuffer.h
/usr/include/t3/widget/t3widget/log.h
/usr/include/t3/widget/t3widget/main.h
/usr/include/t3/widget/t3widget/mouse.h
/usr/include/t3/widget/t3widget/ptr.h
/usr/include/t3/widget/t3widget/signals.h
/usr/include/t3/widget/t3widget/stringmatcher.h
/usr/include/t3/widget/t3widget/textbuffer.h
/usr/include/t3/widget/t3widget/textline.h
/usr/include/t3/widget/t3widget/undo.h
/usr/include/t3/widget/t3widget/util.h
/usr/include/t3/widget/t3widget/widget.h
/usr/include/t3/widget/t3widget/widget_api.h
/usr/include/t3/widget/t3widget/widgets/
/usr/include/t3/widget/t3widget/widgets/bullet.h
/usr/include/t3/widget/t3widget/widgets/button.h
/usr/include/t3/widget/t3widget/widgets/checkbox.h
/usr/include/t3/widget/t3widget/widgets/colorpicker.h
/usr/include/t3/widget/t3widget/widgets/dummywidget.h
/usr/include/t3/widget/t3widget/widgets/editwindow.actions.h
/usr/include/t3/widget/t3widget/widgets/editwindow.h
/usr/include/t3/widget/t3widget/widgets/expander.h
/usr/include/t3/widget/t3widget/widgets/expandergroup.h
/usr/include/t3/widget/t3widget/widgets/filepane.h
/usr/include/t3/widget/t3widget/widgets/frame.h
/usr/include/t3/widget/t3widget/widgets/label.h
/usr/include/t3/widget/t3widget/widgets/listpane.h
/usr/include/t3/widget/t3widget/widgets/menu.h
/usr/include/t3/widget/t3widget/widgets/menuitem.h
/usr/include/t3/widget/t3widget/widgets/multiwidget.h
/usr/include/t3/widget/t3widget/widgets/scrollbar.h
/usr/include/t3/widget/t3widget/widgets/separator.h
/usr/include/t3/widget/t3widget/widgets/smartlabel.h
/usr/include/t3/widget/t3widget/widgets/split.actions.h
/usr/include/t3/widget/t3widget/widgets/split.h
/usr/include/t3/widget/t3widget/widgets/textfield.actions.h
/usr/include/t3/widget/t3widget/widgets/textfield.h
/usr/include/t3/widget/t3widget/widgets/textwindow.h
/usr/include/t3/widget/t3widget/widgets/widget.h
/usr/include/t3/widget/t3widget/widgets/widgetgroup.h
/usr/include/t3/widget/t3widget/wrapinfo.h