The key thing with a regime like Humulin M3, is that it is a mixed insulin - 30% fast acting and 70% slow acting. Taking an extra couple of units to cover a particular drink or food in its short-term peak period, doesn't work with this insulin as you end up putting your slow acting insulin up more than your fast acting. This is therefore unlikely to deal with the early 'spike' from the sugars, but could potentially lead to a bigger drop in blood sugars later on.
I've been Diabetic for 26 years. When I was first diagnosed (10 years old) I was on a Human Mixtard insulin drawn into a syringe from a vial twice a day and injected. This was the same 30/70 mix as Humulin M3. As a teenager I was put onto a basal/bolus regime - 4 injections a day. 3 injections were NovoRapid (before each meal to provide fast-acting insulin to cope with the food being taken), and the 4th was an injection of the basal (slow-acting) insulin Lantus. I stayed on this regime for years, only changing the Lantus to Levemir (a similar insulin but one which suited me better). After a frightening incident in January, it was suggested I try Humulin M3 instead so I am currently on the same regime as you. My control is now not as tight, with more levels above 10.0mmol than I have ever had before. Humulin M3 just doesn't have the flexibility that a basal-bolus regime does for dealing with the day in hand, and the food/drink being taken. I'm going back on to basal-bolus very shortly with my Diabetes nurse's guidance, and feel it is much the better option.
I don't drink at all, as there is enough hassles with Type 1 Diabetes without introducing an additional one. However, some Diabetics do drink and manage it well, so if it is important to you seek the advice of your GP/Diabetes nurse/consultant about how to handle drinks like cocktails and other alcohol, and also discuss with them whether they feel Humulin M3 is going to be the right treatment for you.
It can also seem scary at first, and it does get complicated sometimes, but there is always a way forward!