This might help
List of item values used to set session state within a URL.
Item values cannot include colons, but can contain commas if enclosed with backslashes.
To pass a comma in an item value, enclose the characters with backslashes. For example:
If you store P2_NAME in session state you don't need to pass it over the url at all. You can read the P2_NAME in session state on page 3 as well.