Season 3 does have the highest concentration of crummy eps in it, you can thank Fred Freiberger for that.
Well, really more like you can blame severe budget constraints and the departure of key personnel and in addition; no writers wanting to contribute to a show that everyone knew was in it's last season. I think to some extent Fred gets a bad wrap because he was so limited with what he could do. Truth is it was Roddenberry who abandoned the show. There was really no reason for him to leave other than the fact that he knew there was no future in Star Trek and wanted to move on to something else to generate income. With Roddenberry gone, Bob Justman gone and Finnerman gone; Fred really didn't have much to work with. It is amazing that he was able to produce stellar episodes like "All Our Yesterdays," "The Enterprise Incident," "Elaan of Troyius," "The Tholian Web," etc.