There are 3,6,9 and I've seen 12"
Nothing wrong with doing what your thinking.
If you have nice landscaping you might consider making a platform out of wall pavers

I almost routinely use shorter legs to get even just AC condensers up to limit damage from weed whackers trimming grass around their slab.

There are numerous manufacturers of the legs, not just the 'brand name' (and likely overpriced) ones from the condenser manufacturers.

I have also seen steel platforms that the unit sits on that bolt to the foundation and don't even touch the ground.

As a professional I have installed many heat pumps and they are my favorite unit when installed correctly. One thing that needs to be understood to do it correctly is they only work if air can pass through the outside coil. This means shovel snow away from them after each storm and better.....put on a platform above the height of the average accumulated snow level during the winter. If you get two feet of snow on the ground then put on a three feet tall platform. The cost of making one is worth it in energy saving the first year. If you don't get snow then just six inches to keep off the grass and dirt of a lawn mower is OK, 12 inches only better.