It looks like this file file cannot be accessed by HTTP from your workstation. Could you download it directly by browser or using wget command line utility?
You are getting that error because W3C disables access to a host once it has done too many requests to read dtd's/schema's.
That was caused by a bug in JBoss Tools where we did not always use the cache to get these schemas - those bugs were supposedly fixed in the M3 release (https://jira.jboss.org/jira/browse/JBIDE-4686)
Are they still there if you do a Project > Clean and build again ?
In any case please update to the latest to be sure you got the latest fixes for this problem and if it happens again report it directly in Jira - thank you!