Hi Hatty!
Three hours? Blimey!
A lot is going to depend on how much you can expect the students to know about the subject before you start. Will they have done some preparatory reading? Will you be able to expect some sort of 'intelligent discussion', or will they be totally dependent on you to 'feed' them with the info which they will then discuss?
I would avoid the temptation to write 'an essay' and I'd keep my lecture notes in 'note form'. You definitely want to avoid the temptation to read things out. My notes are often quite full, in the sense that I do tend to write down everything that I want to say/cover (can't rely on my memory!) but I never ever write full sentences. This makes me talk normally rather than droning on. (My students may beg to differ on the 'droning on' bit, but let's just say that it could be worse ).
I think that, at the start, I'd consciously break it up into chunks. So, perhaps, you introduce the subject. Then you get them to discuss some aspect of it in pairs/small groups (depending on size of group) and you ask them to feed back in some way (this will all depend on the subject, of course). Then you respond to the points that they raise, before moving on to the next part of the theme, and so on.
You may find that, as you go on in the course, the classes find a sort of 'natural balance', and that discussion 'just happens' rather than you needing to build it in consciously. But at the start, I'd manage it - again, to avoid the possibility of you 'droning on' (I'm sure you'd never 'drone', btw!) just to fill the silence left by their lack of discussion!
TBH, with a 3-hour slot, I'd be tempted to have two shorter breaks rather than just one halfway through. I do work in 2-hour timeslots sometimes, and I find that they really can't cope with more than an hour without becoming fidgety and losing concentration.
I don't know if you are a fan of powerpoint (I'm not!), but if you use it, keep it for key points, complicated quotations, necessary illustrations and such like. Do not be tempted to put the entire content of your lecture on powerpoint. I've seen people do this, and basically the students just stare at the screen and stop engaging at all with what you're saying. (However, powerpoint fans may come on here and disagree with me on this one. I am not a technophobe, but I am foolish/egotistical enough to believe that people come to my lectures to hear me . If I just wanted them to read my notes, I could just make them available on the internet and stay in bed .)
HTH. I am not checking MN very regularly at the moment, but I'll keep an eye on this thread, so ask away if you have any other questions. And GOOD LUCK!