Elisabeth:
"Eastern" is a very general term for a very broad range of Garter subspecies. If I understand correctly, you got your three Garters 3 months ago. You weren't the breeder so either somebody else bred them for you or they are wild-caught. Either way were you able to confirm a specific locale of origin? That is important. Easterns range from the Canadian province of Ontario to Florida. The latter might brumate 1 month a year while the former might brumate 8 months a year. If you want to provide optimal care you should look into the ancestry of your Garters. Brumation is largely endogenous, and keeping a northern Eastern awake most of the year, or a southern Eastern asleep most of the year, will not bode well for either.

I'll assume that your three Garters all have the same temperatures, humidity, and exposure to the diurnal cycle. That's very unnatural. And the fact that your Chapala is trying to brumate is not at all surprising, given that it has had ~12 hours of day + ~12 hours of night since dinosaurs were pooping in puddles. Your avatar indicates that you are in USA. Regardless of your location in USA you've had <12 hours of daylight since September 21, and each day is getting shorter and shorter. Chapalas would best be provided with an artificial light source during Winter months in USA, but no so for your other two Garters.

Be careful with those Facebook groups. They're fine for photos, but most of those people don't have a clue what they're talking about.

When you say "immediately tried to brumate"... the brumation process for northern snakes starts weeks before they are actually in brumation. What do you mean "food baby she went to bed with"?

Bioactive is not the healthiest nor cleanest. The ultimate bioactive environment is Nature, where snakes defecate and immediately move far enough away from it so that there's no chance of them breathing in the odor. This is why (part of the reason why), that when wild snakes enter the hibernacula they have been fasting for weeks. Once several feet below the ground, in relatively tight quarters shared with several other snakes (sometimes thousands; read about the Narcisse Snake Dens) they can't afford to expel any noxious waste.

The healthiest/cleanest bedding is paper towels. I've been breeding for much of the last 40 years and have tried every substrate available; nothing is as efficient as paper towels in absorbing waste and, more importantly, allowing the location of waste to be quickly determined so that it can be removed. All of those set ups with the plants and the rocks and the water falls and the bells and the whistles... they're all aesthetically pleasing to humans but do nothing to enhance the quality of the snakes' lives.

Nikki:
About finding Garters in 40ish degree or colder weather. That means a hibernaculum is nearby.

40 feet up in a tree is a favorite den spot? When? This time of year they are more likely to be 40' under the ground, at least below the frost line.

When do you watch them come out of the tree trunks; breeding balls way up in the tree? That sounds very odd.