These steps can be done directly in the PSS/E GUI (written for v34, for what it's worth)...
- Go to the Network data tab (top tabs).
- Go to the tab for 3 Winding (bottom tabs).
- Select the row of the transformer you want to copy. (left-click on the left end of the row)
- Right-click anywhere on the selected row and choose Copy.
- Right-click anywhere on the bottom row (the empty row) and choose Paste.
- Enter a unique ID in the Id column (e.g. 2).
- Press Enter.