Hmmm, why just limit it to DSO's really - sorry Sab for your original intentions...
I've got a trio for you. On the Moon! Three craters - Armstrong, Aldrin & Collins. Challenge here is getting the lunar phase right and atmospheric conditions stable enough, particularly with Collins as its diameter is around 3km. Sounds big, and I guess a 3km hole is blooming big, but not so easy to see a 3km hole on the Moon from Earth...
The good thing about this trio is a really big aperture is not necessary. 5" should be enough - just. I've seen this trio only twice, the second time I only had fleeting glimpses of Collins & Aldrin. The other was barely any better. Spy out Collins and you are also very close to the Apollo 11 landing site.