This has nothing to do with the SORTOUT space, so the comments about SORTOUT are irrelevant.
The ICE046A indicates there wasn't enough work space to complete the sort. DFSORT processed 1.4+ million records, but then ran out of work space. Approx. how many records do the three concatenated input files actually contain? What is the approx. average record length (if you know)?
If you add the following to the job:
//SORTDIAG DD DUMMY
to get diagnostic messages, rerun it and send me all of the JES and DFSORT messages offline (firstname.lastname@example.org), I'll take a look at the diagnostic messages to see what's going on. In your offline note, please put "DFSORT" in your Subject line to catch my attention.