I am new to ModX - have created a few sites, used a few modules.
I need to learn/study to be able to convert my other sites (about a dozen). It is advisable to wait for v.3 or will it be simple enough to convert/upgrade?
(Have several fat books on ModX already, but maybe I should wait studying them)
MODX 2. MODX3 will largely use the same concepts.
ModX develops very slowly and stably. There will not big a big shift with ModX3, although I believe a new technology will be implemented, XPDO 3 I think is the key one…
Thank you very much for the responses.
I will go ahead & study/experiment.
Learn Modx 2. Modx 3 will take time to release and then extra to compatible to version 3. So learn Modx 2.x.x and then when 3 release, you’ll not get problems. 
If you face problem then feel free to post from small to big things. Community will help you.