Going from 3low to 3high is the only way I can actually manage 3high. I had to struggle to learn how to get the throws consistent. Barnesy and I have chatted about it in the past, and I think his post here sums up what he's talked to me about 3high:
I just learnt that 3 low to high is more subtle than I thought. Like with so many juggly things, I was overcooking it to start with. I don't try high much, but when I used to try low to high, I would think of it as the siteswap for the transition. But tonight I tried to just creep the height up from low so it became high. I was still doing the same transition (...33345606060...) but instead of thinking about the heights of all the throws, I just crept them up without paying much attention to each individual throw. I spent a while failing before I realised I had to make sure I was ready to catch the first of the slightly higher throws, which was catching me a little by surprise, but once I sorted that out the transition started to work much better than I've had it work before. I still wouldn't say I can do three high, but I had a few short but controlled runs and can see it coming together. This might be the summer when I finally bother to get it solid!
Dave
I spent about 2 or 3 months struggling to get the timing down, but eventually it does come once your throws out of 3low become more accurate. Getting the first two out and changing your arm movement to a high throw motion has struck me as the most important part of going from 3low to high.
Rushing my response because I have to head out, so hopefully someone else can expand on parts I've messed up or forgotten to mention.
-Duncan