Republicans have once again thwarted an attempt by the Democratic-controlled Massachusetts Legislature to approve a spending bill that includes money for temporary shelter spaces for migrant homeless families. An informal session had hardly gotten underway Saturday before the Republican caucus used a procedural process to delay a vote on the $2.8 billion spending package for a third consecutive day. Legislators already wrapped up their formal session without approval of the supplemental budget. Legislation can be adopted in informal sessions but a single lawmaker can kill a bill. The package includes hundreds of millions of dollars to support shelters that are being strained by a crush of migrant and homeless families.