But seriously, why use GTK3 on Windows ?
GTK3 on Linux has a long way to go, my guess is its far worse on Windows ? megatron, perhaps you are not aware of the usual way to make cross platform apps ? if you leave the default setting in place, Lazarus will use the local default widgetset, so, on Linux it will use GTK2 (which is pretty good, stable, widely supported) and on Windows will use the Win32 widgetset, again, just what you want on Windows.
If you want to help track down the GTK3 issues (and more power to your strong right arm) then you would find Linux a much friendly place to do so.
Davo