I had another look at those Scanlan baptisms and unfortunately none of them give a mother's maiden name. Neither are there any Scanlan marriages in the St Helier registers, although I forgot to look in the garrison register - next time!
If Thomas, John and James had been siblings I would have expected their names to have appeared somewhere amongst the various godparents, this is not the case.
A baptism at Elizabeth Castle certainly does imply a military connection, although in the case of John and Hannah it would have been unusual for a regiment to be stationed here for 7 years, they usually came and went quite quickly.