You can subclass TreeItem and override TreeItem.insertItem(int beforeIndex, TreeItem item) to eliminate the margin. The margin is currently set in TreeItem.insertItem() line 575.
The 3px padding seems to be set by the TreeItem as base padding and then Tree increases left/right padding depending on your TreeItem image width. See TreeItem.TreeItemImpl.initializeClonableElements(), Tree.showLeafImage(TreeItem item) and Tree.setImages(ImageAdapter images, boolean useLeafImages). Not sure if you can change this behavior to your needs without copying the whole Tree/TreeItem class into your project so you can freely modify them.
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