: M3 contains 30% regular insulin: a fast insulin (but really isn't that fast) and 70% of isophane insulin: which is a slower acting insulin.
Novomix 30 contains 30% aspart : a rapid insulin (and quicker than regular) and 70% aspart protamine, the protamine makes this part slower.
The result of this is that Novomix 30 starts working more quickly and is at it's peak earlier than M3.
(peak action the time when the insulin is 'strongest')
You can compare the times here:
http://www.endodiablibya.org/ldcg-app3.pdf
It makes sense to try this if you are having hypos between meals, as the rapid bit will be out of your system earlier. Since it also starts working more quickly then it might also help lower glucose spikes just after breakfast and dinner (assuming that you take your insulin before these meals)