That means they will plea bargain for a lesser charge. I'm sure that 3rd degree arson is still a felony. Then of course, they may get off easy by pleading guilty to misdemeanor criminal mischief. Wouldn't surprise me.
I don't know much about federal prosecutions, so can't speculate on that.
Here's a little more about the "pranksters" in an AP story that I guess I can't excerpt:
http://www.ledger-enquirer.com/mld/ledgerenquirer/news/local/14050050.htm
The AP story says they "could get" 5 years for EACH of the 9 fires = 45 years possible.