I did create a triple grass core, using sinistcha, rillaboom and hydrapple (which I made an RMT on, but someone responded to it with something irrelevant and thus nobody else responded and I absolutely was not annoyed by that), and I have to agree with you. I used personally great tusk, kingambit and kyurem in my other three slots. An ice resistance is key, but you most of the time have to find grasses neutral to it's normal weaknesses. So that's why I chose sinistcha and hydrapple, because they are neutral to poison and fire respectively. The rest of the core should resist or do well against steels, fires and poisons.Well, my point is actually that I think we should all stop playing moderator and trying to control what people say in the discussion.
Grassy Glide comes with Rillaboom for free, so it's a little bit unconventional to have it on something else unless you are going double Glide. The problem with double Glide being there are like 7 resistances to Grass. So you would want the team to cover for that somehow. Ice, Ground, Fire, and sometimes Rock are all types that can hit multiple Grass resists. Fairy attacks are also decent for hitting hard neutrally and stopping dragons that aren't Gouging Fire. Dragons in particular are a huge problem since they are very common in the meta. You would also need an obvious Zama answer because physical attackers.
I tried something somewhat similar with Rillaboom and Teal Mask Ogerpon for the speed boost awhile back. Before the DLCs. It was very mid because there were too many stacked resistances to Grass Spam. It's probably worse these days I don't think I would even try something like that now with all the Gouging Fires and things making it even harder these days. But Waterpon at least has handy Water STAB. So it's not all bad.
In the case of Waterpon specifically, there are a lot of mons that outspeed it and revenge kill it. Grassy Glide would give it the option to hit those mons and at a speed tier that is higher than most of the priority users. E-Speed and Comfey still ignore that, but it's still decent against a lot. Waterpon Tera is also nice to have against Weavile Ice Shard and Cinderace Fire moves. Probably not Roaring Moon, though.
For complimentary attackers, I'm just gonna state the best options with each STAB that I listed earlier. For Ice attackers, you want Weavile or Kyurem. You could also maybe get cute with triple Grass spam and Triple Axel from Protean Meowscarada. This stacks a ton of weaknesses, but you do have a couple ways sort of around that and it's a more offensive core anyways.
For Fire, you want Gouging Fire, Volcorona, Heatran, or Cinderace. You can potentially pick more than one if you have room and you want more firepower. Pun intended. This is because Waterpon forms 2/3rds of an offensive Fire/Water/Grass core. But again, beware of Dragons.
For Ground, the best offensive grounds are Tusk, Treads, and Lando-T. You might want Lando-T because it gives you a ground immunity and a special attacking option. Something cheeky you can do with Heatran on the team is stack fire weaknesses like Treads. But maybe it's a bit too telegraphed.
Rock type attackers are tricky. Iron Boulder really isn't very good in most cases. You don't want Cornerstone if you have Waterpon. Glimmora is decent hazard support mon, but it's generally not used as a primary attacker. You can still make the set more offensive, though. Generally, I would skip this type as a primary focus in OU.
You can't fit too many complementary attackers next to two grass mons since you still need room for whatever bulk and/or support you are running. So focus on one or two primary Ground, Fire, or Ice attackers (or Meow if you are feeling cheeky) depending on what you are going for and what you have room for. You should try to fit in glue mons that also have Fire, Ground, Ice, or Rock STAB and/or coverage on them where you can. I think this would be the best way to make grass spam work.
That's how you do triple grass cores, though mine was created when rain was dominant.