Hi S.. I am a step mom to two son's who are now 21 and 20 and have two daughters who are 8 and 6. It sounds like you might be expecting too much of the three year old. I can assure you that with discipline or not, the three year old would still be telling most of us no. My six year old was emphatically telling me no this evening. As parents we need to repeat every lesson thousands of times...it is a bit dull and majorily repetitive, but necessary. Spanking is not something that ever really worked for us. It just teaches them to be beligerent. I think we spanked one of the girls once and the other one never (she just thought she was going to get a spanking). The best way to get a three year old to stop doing something is to distract them with something that you think is okay for them to do. Like if he is pulling a lamp down, firmly say no, then walk him to a toy or book or whatever. You will need to do this every time until he stops, or move the lamp. Or whatever the scenario is. Parenting is hard. Step parenting is harder. I have had many jobs that were demanding and stressful, but none as tough as being a parent. Being a good parent is very hard. Being a great parent is nearly impossible. What is possible is parenting with love, understanding, compassion and forgiveness with our natural and step children.
Also, moving between homes is tough for much older children, so it is probably confusing and stressful for these little guys. The good news is, but the time your three year old step son is out of the challenging age, you three-month old will be entering...so thankfully only one toddler at a time!
Enjoy getting to know all of these young boys..they'll bring you much joy when they are older!
CK