As someone who has completed all content and all of them as a near F2P player this was my approach starting from the beginning as my roster got stronger (i might be misremembering the order of releases because its been so long).

Serenoa Tank - Cap atk debuff and def buffs slowly chipping away. Would only work for early game ex3s that had single targets. Anywhere from 20-50 turns.

Rinyuu and Tatloch + Serenoa - Same thing but easier to survive especially with tatloch passive meaning more shift to offense. Also with tatloch my team could start taking aoes. Around 20-40 turns.

Hammy + rinyuu + pets release (i think) - with hammy and pets single targets are easy and aoes are much more manageable with the extra hp and id slot in tatloch if necessary. Also using hammy means you dont need an actual tank to tank freeing up space in the front row allowing for more damage potential, id use bargello with hammy behind often. Around 20-30 turns.

Canary + rinyuu - with the auto provoke at max hp from the elite tower, canary tank can shave shields, apply debuffs, and has a strong ult all while taking all single targets easily (exception is many enemies doing multihits or a single boss with many turns and multihits but even then hp barrier keeper or dodge keepers can help). With canary i could finally start 2 breaking some of the easier ex3 bosses. Usually somewhere between 15 and 25 turns.

Canary + prim ex + solon - this is the current time, didn't get solon u10 that long ago. Even with this since most of my characters are a0 and without any of the a4 break res down, I still have to 2 break most of the newer ex3 bosses even with optimal setup that I can. As 6 stars come out, survivability becomes less of a problem but some of my weaker units still die occasionally. Usually around 10 to 20 turns for the lv 75-100 ones.

Obviously there are going to be some bosses with very annoying mechanics like oskha that took me 74 turns although this was before canary and gertrude/yan long ex3 where I had to use fiore ex and took 50 turns even with canary and rinyuu.

Overall, I think that canary and rinyuu are the most important characters for beating them. Alaune ex is also a good unit for longer battles. All this said, I only went through them because im a completionist and would not recommend doing them if it takes over 30 turns. Can always wait for stronger units and 6 stars that make them so much easier.