The arboretum is still there, but there are fewer tables and chairs and unfortunately many of the tables are broken. There is an elevator to get to this space. It is my favorite place to relax during a long layover, but the seats do fill up. You will often see crew members spending their layovers in this area. I agree with you that the yoga room is great too, be courteous to others who might show up to use the room also.

Another area I like that is not a lounge is the high L gates in the new extension (L20 - L24). A bit of a walk but that area is usually pretty quiet compared to the rest of terminal 3.