For the cauliflower, I don't really tie the leaves I just use a few rubber bands to pull 2 or 3 leaves together over the middle of the plant. You can undo the rubber bands each day to check on its progress.
You harvest cauliflower when the head stops growing larger and before it starts moving apart. You want the head to be tightly packed together. It is okay if it moves apart (sideways) a little before you harvest it, but once it does this, it needs to be harvested as soon as possible. I let mine grow until the first signs of it separating. At this time, it wont get any larger. This may be hard to understand; I can upload pictures if you don't understand what this looks like.
Broccoli is similar to cauliflower. This time of year, you will have to watch the broccoli even closer than the cauliflower. It tends to separate and become inedible more quickly than cauliflower does. Your broccoli may also not be as large as the cauliflower. Like cauliflower, you harvest broccoli once it stops getting larger, but before the stalks separate and the yellow flowers open. If you see a little yellow showing on the buds, harvest immediately or it won't be good.
I'm not sure on the cabbage; I haven't grown it (yet).