Hello!
To get the best experience with, I don't recommend you to use a Blog CMS because you must have to develop some functions (forms) like "Ask a question", a specifical theme (for a questions site), and a good commenting system. So, one down vote for Blog CMS.
A forum "CMS" has many features. You can see these fetures here, on this forum. A forum CMS is a big application so it consumes many resources (it depends of who builds it and how it is built - there could exist forum applications whch uses a bit of resources, but the majority does not). For example, if you will start with a shared hosting, after some time, it will go down and you will believe that you have too many visitors. NO. You haven't many visitors. The shared goes down because of your forum application which makes too many db queries or which uses much memory and/or cpu (it depends on where are you hosted on too - a hosting provider could allocate more resources than other). Another down vote for Forum "CMS".
A Q&A platform is usually lightweight (it depends of who builds it and how it is built). It hasn't as many features a forum, but it usually has only tags, logins system, register system, order by tag, order by newest, add question and simple answer question as features (not as many as a forum "CMS"). But it could be developed as well as a Forum. An up vote for Q&A sites.
I couldn't select for you what to use... it depends on your requirements, and what to do with the website...
If you have any further question don't hesitate to tell me.