Hi, being 5 1/2 months old - it sounds to me like they may be ready to start solids! Try giving some rice cereal to them  - I am starting with my daughter today due to a similar problem - hope it works!!

You pretty much have to feed on demand because like you said, you get cranky babies.  But as far as not waisting formula I would suggest breaking it down. Maybe give 3/4 and save the rest that way if the boys want more you can give more but if not, you at least get to save the untouched portion until the next feeding.

As far as the 3 hr schedule, my girls kept to that until they were about 7 or 8 months and then we went to the 4 hours (I was breastfeeding though).