1) Blaze of enemy killings interspersed with religious incantation
2) Door closing (literally) on Kay as she is shut out
3) Michael ordering the killing of someone close to Connie that later makes her hate him more
1) the third one also had a killing montage but too bad that damn opera music made it sucked and it was too damn long
2) in a way Kay closed the door on Michael (just not literally) and officially shut him out of her life
3) Connie actually ended up killing someone close to her
Also parallel beginnings:
1) Wedding out front, lots of "business meetings" behind closed doors
2) Festive occasion out front, lots of "business meetings" behind closed doors
Not sure about (3).
Another parallel -
In (1), Solozzo makes an attempt on Vito after meeting resistance to his plans in the drug trade, whereas the next in line, Sonny, would in theory be more co-operative (hot for his deal etc)
In (2), Roth makes an attempt on Michael after meeting resistance to his dealings in Cuba, whereas the next in line, Fredo, would be more co-operative (no 'in theory' needed this time!)
Both attempts fail
And even if they'd succeeded, both attempts would have backfired
In (1), Sonny would never have gone in to business with the guy who killed his father - why would anyone?! The all out war Sonny escalates is, I imagine, pretty much what would have happened had Vito died in 1945.
And in (2), can you really see Fredo being made Don?! Granted, Michael would have been dead and so couldn't formally put Tom or anyone else in charge, but Fredo taking over would have seen all Michael's muscle and business and legal support walk away, or the Corleone family crumble to the point where their fully supporting Roth's business would have meant nothing.