In jboss each datasource needs its own pool. Why would you want to do otherwise? If the connections are equivalent, why not just use one datasource?
You should be able to use 3 *-service.xml files with no problem, or put all the mbeans into one file. I'd also recommend considering using 3.2 and the simpler *-ds.xml files. With the *-service.xml files you need mostly to make sure that all the (9, for your 3 datasources) mbeans have different names.