They are all "diaries" written by our favorite wimpy kid, Greg Heffley, who seem to be naughty deliberately but carries it off with an innocent air. These diaries list his many misadventures - quite a lot of them involving his brother, Rodrick, and his best friend Rowley Jefferson.
In The Third Wheel, a Valentine's Day dance has been planned at Greg's school and that starts a scrambling among the students for dates. After a lot of searching, manipulation, and failures, Greg finally gets a date but by the end of the night, a lot of things change in strange directions.
That strange twist is the meat of the plot for the next book, Hard Luck. It has driven Greg and Rowley apart, and in his new friendless state, Greg finds that nothing seems to quite work well for him. He finds that not having Rowley to walk in front of him to school meant he would end up stepping on dog poo more often, not having Rowley to carry his bag meant he would have to resort to taking a trolley bag to school. In short, his life sucked and he spends most of year depending on a Magic 8 ball to decide whether he should eat breakfast or if a certain question's answer was true or false.
I borrowed these books from the good old library.