I take fish oil and niacin to lower my triglycerides, can i also add garlic tablets?

You can but: According to most clinical studies, garlic was not proven to have any impact of cholesterol. All the best!

You can: But i doubt it will work. I've not had any luck with Niacin and modest with fish oil, unless it's a prescription strength. Best way is weight loss and avoiding sweats, easily absorbable carbs, etc. If you have hereditary hypertriglyceridemia, none of the mentioned meds will work effectively.