Humulin M3 is a mixed insulin, so it makes perfect sense that you're getting too much of one and not enough of the other unless you would eat exactly the same every day at exactly the same time, had the same amount of exercise every day etc.
Most T1's use two insulins, a basal (long acting) and a bolus (short acting). So they can use the basal to keep them steady in the absence of food, and adjust the bolus for each meal depending on the amount of carbs.
It means a lot more work than using a mixed insulin, but also a lot more flexibility and more stable BG if you take the time working out how much you need for what.
Many T2's nowadays also use this basal/bolus system, although many of them are on a mixed insulin like you do.
If you're interested in starting this, by all means tell your diabetes nurse!
This may be interesting for you if you decide to swich:
https://www.bertieonline.org.uk/
It's aimed at T1's (no such thing for T2's yet, sadly), and as I understand it can be a great help in learning to adjust insulin doses to your food.