I have a Troy-Bilt lawn mower, TB360 (PDF manual). It has 6 height adjustments for the height. I measured the height of each one on a flat surface using a shortened wood paint stirrer and notched the blade at each height. The top two height levels are 3.5" and 2.875" respectively. I'm trying to get in between those, closer to 3" or a tad higher than 3. Are there any ways to adjust in between these heights? Do they sell "custom" blade adapters that are a bit thicker and therefore would push down the blade when set at the tallest (3.5") a bit more so it comes closer to 3.25" or 3"? Is this a thing?
Most commercial mowers and many manufacturers, now highly recommend, adjusting the blade so the rear is one adjustment "higher" than the front. With a properly matched blade-to-deck and rpm, this will create the necessary "vacuum" to lift the blades of grass in the (rear) and cut them evenly on the lowest (front) end. And I believe, if you're at 3" inches in the front, and the next higher up in the back, you'll be where you want to be due to the ever so slight of an angle.
DO NOT sharpen your blades to a "sharp" cutting edge. This is bad advice from people who know little.
Doing so will only destroy the blade before its time. Consider this. A blade running 3600 rpm, with a razor sharp edge on it? What's going to happen to that edge the SECOND it hits anything, including grass? 99 percent of the cutting is done by less than 5 percent of the outer edge of the blade. A "sharp" edge will assure immediate chipping of the edge - opening the door to rapid wear from the tip inward, and you'll quickly have a blade that's unfit to even re-sharpen. You should have what many of us call a "smooth" sharp edge to it. You can't cut your skin with it while it's static, but it'd chop your leg off at nice and smoothly at 3600 rpm.
P.S. I highly recommend ignoring miss stormy advice at all cost.
Seriously, the higher the end mow the better! Go with 3.5", my goodness! The higher that grass is the more vigorous your lawn the fewer weeds will germinate or be able to compete. You are so lucky to have purchased a wonderful mower that actually allows these heights! Good for you.
The higher that grass is mowed, the less it grows per week and you'll only be cutting the tops of your grass, thus even less stress! Less debris (I would still bag my clippings great stuff unless on your lawn) from each mow. I am very glad to hear mower companies are raising 'the bar'. Most people have to have their mower height ability raised just to be able to cut 3"!
The higher one can grow their grass without it falling over (to compromise photosynthesis) the healthier your lawn crop. Most grasses the ideal height is 4"...but 3" has been shown to be almost as good and is far easier to sell to people who want a golf course green. That was what most mowers were designed to do, to accommodate the notion of a lawn looking like a golf course green. At that height, lawns have to be fertilized constantly, lots of herbicides for weeds, having to thatch and fuss and still their lawns would never be as beautiful, vigorous, as green as lawns cut at least 3" high no lower. Go with 3.5"!!
Lucky you!! Get an extra set of blades, learn to sharpen and how to change your blades. Sharp blades make a big deal for the look of your lawn. A little too dull of blade will shred the tips of your grass blades giving your lawn a dusty look. As well as allowing disease to possibly be introduced. Get to know your mower fix it guys! Have them show you how to change filters. Purchase extra filter for the gas and air. Brown nose enough they will show you any and all minor repair/maintenance you are able to do without having to have your mower, your blower, your weedwacker maintained by them at a cost to you. I am the biggest brown noser ever and boy did it pay off! You will love to mow your lawn. In fact, to mow it twice a week is just fine and dandy! Always change your directions each time you mow and you will have the best lawn on your block!