In the same way that you are adding a single button, you can also add multiple buttons but for those buttons you will need an additional container. You can try out something like below:
col.setLabelProvider(new ColumnLabelProvider() {
@Override
public void update(ViewerCell cell) {
TableItem item = (TableItem) cell.getItem();
Composite buttonPane = new Composite(getTable(), SWT.NONE);
buttonPane.setLayout(new FillLayout());
Button button = new Button(buttonPane,SWT.NONE);
button.setText("Edit");
button = new Button(buttonPane,SWT.NONE);
button.setText("Remove");
button = new Button(buttonPane,SWT.NONE);
button.setText("Deactivate");
TableEditor editor = new TableEditor(getTable());
editor.grabHorizontal = true;
editor.grabVertical = true;
editor.setEditor(buttonPane, item, columnIndex);
editor.layout();
}
});