I see you are a Samcedes shipper, so I expect you to disagree with me on this point. :) What I saw set up in early S3 for Mercedes was a strong story, paralleling Rachel's story, with Mercedes coming to find her way to claim the spotlight (something she's wanted since the Pilot), and with Rachel struggling with the fact that she isn't the only star. They even set up parallels w/their boyfriends, with Shane winning the scholarship over Finn. But then it turned into a Mercedes love triangle.
Mercedes story about claiming the spotlight was mostly featured in Asian F. The third episode of the season.
The fourth episode, she was there for a couple of seconds to recruit Santana and perform a TT number. The fifth episode if you blinked you would have missed her. You can only see her for 0.0006 seconds among the audience for the West Side Story premiere. In the the 6th and 7th episode she took a backseat to Santana and the elections, only there for TT numbers and have one scene to be elected captain of the Troubletones .
There was no Sam, no triangles in episodes 4-7 and yet there was no strong Mercedes storyline in sight. So how Samcedes can be blamed for them dropping something that was only shown in episode 3?
Shane was barely a blip in viewer’s radar, they never developped him in the 7 episodes before Sam’s return.
If it wasn’t for Samcedes, Mercedes would be either absent or twiddling her thumbs along with Tina and Puck during the second half of the season.