Stringers are a tricky topic. I have some decent experience with both Tri-Stringer and Wellstring and they're essentially polar opposites with entirely different gameplans and win conditions. Tri-Stringer's focus is snipe shots, with its AoE as a secondary feature to flush enemies out from behind cover and maybe deal some chip damage. Meanwhile Wellstring's focus is walling out space at a distance with explosions, and sometimes stacking its arrows on the same spot to try and get splats. And also charging in as a pseudo-midline.
Tri-Stringer is best for players who value range above all else and don't want to play chargers. It's harder to use than chargers due to its shot spread, but has higher flexibility with its falloff shots. It can use its partial charge AoE to flush out enemies behind cover or buy an extra fraction of a second to escape danger, but the focus is fully charged direct hits. Even if an enemy is already chip damaged by Killer Wail 5.1 or Super Chump, it's better to go for full charges, since they're just more consistent for dealing meaningful damage. The weapon is ultimately very simple compared to Wellstring.
Wellstring is better for players who prefer scrappier, more flexible anchors. It has better paint and AoE than Tri-Stringer, but no ability to land long-distance snipes. It ultimately plays similar to Explosher, with about equal difficulty; it's far more ink efficient than Explosher but has less consistent damage on its AoE. Wellstring has pretty much all of the tools an anchor needs to be able to position more aggressively without dying: A close-range one-shot, a get-off-me tool (in the form of its partial charge explosions), and a kit that gives it a panic button and/or lets it locate any would-be threats and keep them at an arm's length. It works well both painting and walling out space at a distance, as well as pushing ahead with its team, though it's only likely to get splats in the latter case.
Outside of being an anchor, a stringer's goal is to wall out space with chip damage; Tri-Stringer sometimes picking off enemies with directs and Wellstring sometimes pushing ahead with its team. There isn't really that much reason to pick Tri-Stringer over a charger (Pencil), but that's true for most backlines as of now. Wellstring is good for comps that benefit heavily from its Point Sensor spam and constant location, though its vanilla kit is generally not worth picking outside of solo queue Rainmaker. All three are difficult in their own ways, but Tri-Stringer is the most solidly backline weapon of the bunch, meaning it has the strictest positioning requirement. Wellstring is the easier of the two anchors.
The main reason you bring a stringer to Rainmaker is to focus on the rainmaker; both of them can shred its shield up close and at a distance (with Wellstring making use of its falloff to do so), and its carrier is an easy target for stacked explosives. Just remember that if a teammate picks up the rainmaker, you have one less player between you and the other team. Try to pick it up yourself whenever possible; its playstyle it pretty similar to a stringer's anyway.
Not sure how helpful this was, but it's what I've learned from playing stringers here and there over the past couple of years. Hopefully you were able to glean something from it. Have a good day!
