I'm glad that this thread showed up, I just recently had my first 3rd plateau experience and wanted to kinda share it

I haven't been using DXM for too long, and my usual dosage is around 300mg for a good second plateau trip. After trying that a few times I felt the urge to do more and see what other kinds of things I could experience on the upper plateaus, because although the lower plateaus can be nice, they just lack the dissociation that I was looking for

So, I walked down to the store and picked up two bottles of robogels, each containing 300mg of DXM at 15mg a cap. Being the complete dipshit I am, I decided to take the first bottle and head on over to my grandparents house to lock myself in their guest room and use their computer's stereo system to listen to some relaxing music while tripping. After arriving at their house and starting to feel the effects, I decided it would probably be a good idea to go home if I was going to take the other bottle (it was about 30-45 minutes in between the bottles). So, since I live close to my grandparents, I downed the second bottle there with some iced tea and began my walk home. I didn't really begin to feel extremely inebriated until a few minutes after arriving in my room. I wanted music for my trip, but I realized I didn't have the time to set up my laptop because I'd be tripping pretty hard soon. My vision had already began to become choppy, as if the framerate had been turned down or something. The same effect occurred with my sense of sound, and I found it rather fun to play with objects and listen to the choppy/skittery/drony sounds they made. As I sat on my bed with the shades open, I noticed the light seemed very intense and brighter than normal, very warm looking. This is where I really begin to forget things, I can only remember curling up on my bed and closing my eyes, because my brain had pretty much given up on vision. Some objects seemed enormous while others seemed too small, and whatever object I focused my gaze on looked extremely magnified. When you have this much DXM in you (im not sure about other peoples experiences with 600mg, but I'm 130lbs and took it on an empty stomach) everything is how they say, "flanging". I like to say, traily and choppy :D
At this point, the closed eye visuals began to get reaaally weird, and I also felt increasingly detached/dissociated from myself. I remember seeing two things as I was curled up, eyes shut on my bed. The first thing I remember seeing was a black sun. It appeared to have been drawn in black ink on a white background, it had many flares coming out of it, and a smiley face on the middle I believe. I remember seeing this and watching the trails coming off of it, almost as if wind was blowing the flares from it, and leaving little tendrils to follow behind. The next thing I saw, was me. But not my body. The best way I can describe it is, I knew that this thing encompassed everything. This experience, the universe, me, everything. I saw in front of "me" a green rectangle, dark green and slightly flattened. There was also a flanging effect that accompanied that, but it was more hazy and intense than before. The best way I can describe it is, it looked as if it was re-entering earths atmosphere, and there was a coma or tail behind it, and also around the edges of my vision. Anyways, even though I was looking at is as though I was separate from it, I actually felt as though I was it. But at the same time, not. It was a very unusual feeling that I don't think I can describe. Everything was so dream-like and unreal, I felt as though I was nothing, and everything at the same time. After two hours(?) or so, I descended to a lower 3nd plateau state, and proceeded to open my eyes and mutter things to myself for quite a long time. I don't remember any of what I said, I only remember knowing that I sounded crazy, but I didn't care. I lied there and stared at the ceiling feeling wonderful and warm, I could feel the smile on my face as I said whatever came to mind. It really felt great
Anyways, after that, I remember nothing. I might have fallen asleep or something, but I'm not sure. I just know it was peaceful and warm. I felt like I was nothing, in a dream, and that made me feel comfortable. So, that was my experience, not sure if that is going to be helpful, but I hope it can be in some way. I will certainly do it again, soon. And certainly higher doses later. I just have to learn to stop doing it so much :D I've never felt nauseous or bad during any of my trips, except for some mild shaking during my first trip, I found that enjoyable though, I couldn't stop smiling at it.
My analysis is, DXM can make you feel a whole range of things, mostly good things in my opinion. In the lower plateaus, I sort of feel crazy, but calm, and at peace with it. In the 3rd plateau, I feel very disconnected from reality and myself, and that feels good to me.
Sorry if there are many mistakes in my writing, I work a night job and I just got home a few hours ago, I'm pretty tired.
Hope that could give you a little peek into what this amazing experience feels like

(also, I understand that people experience many different things at this level, and all react differently, many hate it, some are indifferent, some like it. I'm glad to be in the latter category)