I don't have a recipe using those ingredients, but I'm happy to throw in my two cents. I think you have a couple things to consider. Historic and modern mead recipes come out pretty much the same at the end. You're combining honey and water, and fermenting it. I would just decide what type of mead you want to make (i.e. dry, sweet, etc.) and then start putting your recipe together from there. Decide the batch size, amount of honey, and yeast you're going to use. The flowers won't really affect the fermentation. Unless they have some wild yeast hanging out on them
The next thing to determine is how you want to use the flowers. Again, here you have a couple options. You can boil or steep them before adding your honey and fermenting. Or you can add them after fermentation has completed. If you're adding the flowers after fermentation, I would still recommend steeping. You can siphon off a small part of the batch, heat it up, steep your flowers, then cool it back down and add it back to the full batch. I tend to prefer the first method and steep at the beginning because it's less playing around with your mead once fermentation is complete. The key here is to add your flowers to a hot liquid so any wild yeast or other unknowns on the flowers are killed.
Similar to what you're doing, I've made several meads with heather tips. To determine what amount to use, I initially ran several tests by steeping heather tips in hot water (around 180° F). I tried this with different amounts of flowers for different amounts of time. Basically making a bunch of heather tea variations in half gallon batches. Then I tasted them to find out which ones I liked best. With the ones I liked the best, I dissolved in a little bit of honey, to see what happened with the flavor. I don't have my recipe handy, but I think I ended up in the neighborhood of 1.5 - 2 oz of heather tips per gallon of water, steeped for 7 minutes. The flowers were dried though, so you'll likely have different results if you use fresh flowers. I also know that lavender will be much stronger than the heather tips. Not sure about the rosemary and bee balm.
Because you're using several different flowers, you could make up some sample tea batches for each, and try different combinations to see what works best. You may find that all three together don't work very well. I hope that helps a little. I like the batch idea. I've been meaning to try a lavender mead myself but haven't gotten around to it yet.