Are you copying and pasting text from elsewhere without using icon 5 or icon 6 in Row 2 of the visual editor? See: http://en.support.wordpress.com/visual-editor/#pasting-text
Go to Settings > Writing and select “ ___ WordPress should correct invalidly nested XHTML automatically” and then scroll down and click “Save Changes.”
Posts > All Posts, change all your latest posts to Draft, click Update, change them back to Published, click Update again. (You can edit all 3 of them at once: tick the square in front of their titles then select Edit from the Bulk Actions dropdown and click Apply.)
If that does not work then please post an active link starting with http:// to a post or page that has an example of what you are referring to on it that we can view.