Hello Phil,
As Bob commented, there is no additional lines within (below) a staff text line for lyrics as you want. Adding on from his suggestion, you can simulate a 3 line text by leaving the first system's text where it is, the second system's text up halfway, and the third up fully.
Another thought is related to viewing your lyrics as the iReal b plays in the background. You could save your lyrics in a PDF, then use a PDF viewer. See:
http://forums.irealbook.net/showthread.php?t=1124
We may create some kind of lyrics page facility in the future. Thanks for your question.
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