i had a similar problem, my server has 14 instances
just check the operating system swap space and RAM
try to distribute SGA of all the instances so that it can fit in the O/S RAM .
i.e reduce SGA of one instance and increase SGA of other instance all the instances would start .
sum total of all teh SGA's may be exceeding the operating system swap or RAM area .
hope fully this will fix
alternative , just shut down one or two instances and start the one which is not starting .