1 netflix hd feed can easily wipe 3.7m. Most times it is in the 6m range. Both netflix and youtube though can dynamically adjust the amount of data they request from the servers. Your issue may be something different in the browser or maybe the flash player. Hard to say both these companies are not very good about sharing info on how they determine bandwidth.
Which traffic gets sent or dropped and which has priority has nothing to do with your router. The ISP is in full control. Pretty much when the connection gets full they will drop traffic randomly. This means they will likely drop the same percentage from all open sessions...but there is no way to say for sure since it is random.
Your router can do nothing about this by the time it can do anything the data is already discarded and its not like it can recreate it and drop something different. Pretty much your router only has the ability to control what it sends and you likely are not exceeding your upload bandwidth.