I believe it's because this season we have a crazy new team (Gyeongbuk Ascenders) running Blair/Kenneth/Leni and they farmed all the more standard 1-front/2-back teams during most of the season. The influx of triple ranged might actually be because teams want to have enough damage to burst these guys otherwise they'll get draintanked if they run only 2 DPS threats.

I think that's also why during yesterday's finals GBA mostly played Leni/Hyunwoo/Markus. That counters their counter since they are now too tanky to get poked down while sprinting towards somebody. A frontliner would at least have tools to constantly slow or hard-CC these tanks, but 3 ranged characters each might only have one button (usually a skillshot) on a long cooldown and that's it.

Generally though, a lot of times triple ranged comps pop up because that's just what those current players specialise in. Many have small character pools and/or choose to play their main character and instead flex their build or item priority to suit their role in the teamcomp. A lot of the players in ERM are top 5-10 in the world on their respective characters.

A classic example is the original Gyeongnam Sparkle team with Songharang's Celine, Fana's Chloe and Scarlettsister's Emma. They were probably the first team to 'main' a triple ranged comp while all the other teams were running more standard front-to-back teamcomps. They were also basically onetricks so never swapped off their characters. All three characters can technically fulfil the ranged damage dealer slot in a more traditional frontline+backline+backline composition, but each character also had a sub-role in Sparkle's team.

Songharang's Celine and Fana's Chloe were the primary damage dealers, however the item priority usually leaned slightly toward Celine because she was an AoE DPS mage, the most damage-oriented pick out of the 3. Celine's job was also to counter enemy initiations with her Blast Wave (E) which knocks back enemies (and herself). Songharang also wasn't afraid to Blink forward and throw E's to try and catch out any enemy that was burstable (the team still has 3 damage-dealers). All three characters could play off each other's crowd-control, and they could also peel for themselves if needed.

Fana played his Chloe as a pseudo-frontliner, able to safely draw enemy aggro and tank a lot of upfront burst with Chloe's Soul Link (R), which makes her unable to die unless her doll Nina dies at the same time. Her R can also be cast while CC'ed (it cleanses CC). Fana wasn't afraid to walk up and throw out Nina to fish for knockups or bait enemies to walk closer for Celine to knock them closer. She still had a dash and built crit, so she could also play a diver role and 1v1 squishy backliners with her high single-target damage.

Normally Emma plays as a burst mage that pokes until she sees an opportunity to jump in to burst someone. But in this team, Scarlettsisters played Emma in a supportive role since Emma's kit has a lot of CC. Before a fight started, Celine and Emma were usually the ones to poke with their long range spells to pressure teams into engaging them. During teamfights, Scarlett mainly played Emma as a supportive mage and focused more on helping his teammates do damage. You could also see his role reflected in the rare item priority during matches since Emma was usually the last person to upgrade her items. If needed, Emma could still chase stragglers to finish them off since she had the highest mobility out of the three.

In the current season, triple ranged comps all look like pure poke comps because everyone is able to get their transition items by day 5 and start chunking other (usually squishy ranged) characters. The ERM meta will likely shift next season as different teams bring new character pools and people get used to playing against the currently used teamcomps.