The Erebor 3 mans are definitely more about tactics than just having the trinity - as someone else already said. I do think having a tank/healer/dps set-up makes it easier, but they are doable with off-healers and off tanks.
Once you have the mechanics figured out - they will be much much easier.
For Scuttledells for example - for the first boss - the trick is in how you pull the groups. Dealing with them one at a time makes it super easy (once you get it down). Etc. I'm sure there is at least one thread on how to use this strategy. Same with the last boss - look for threads here or ask for someone in your kin or glff who can run it with you and show you how to do it.
Goblin Town is tricky - it's a longer run and you really need a target assist and to have a plan on how to handle mobs, pulls and bosses. I know there are guides/tips because we did a lot of looking around for tips on some of the end bosses (three in succession) to get it down.
This group of 3mans is definitely about strategy more than just brute dps and heals. Which is really good prep for the Erebor raids. In most cases these raids are more about mitigations and coordination than straight up highest morale, highest dps and highest heals. The fights are long and require everyone to know their role and how to work together.
Best of luck to you - once you get these 3 mans down they are actually a lot of fun.
At home on Nimrodel for 4 years.
Hadleighy/champ/100 (main), Brynnraen/guard/100 (retired main) , Zarefina/mini/100 (solo), Hazriel/LM/88, Rachlarien/RK/83, Brynreign/Beorn/71, Rudbekia/burg/34, Aysawen/capt/36, Haddley/hunter/16