Without getting too in depth they are your rhomboids and middle fibres of your trapezius. I'd also aim to strengthen your scapular depressors namely the lower fibres of your trapezius too.
Now before everyone jumps on my back, rounding of the thoracic is ok if your going for a "true" 1rm. That means 1 rep, deadlift gone for the day. But I believe maintaining thoracic extension and curveture on anything 3rm and up is important. If your rounding every time you dead you've got issues to address.
If your doing 5/3/1 that means your 5's and 3's should be done with good form and a little rounding on the 1 is ok but remember that program is designed so you only do a real 1rm every 3rd week out of 4 but even then you might manage a 2 or 3. The way the percentages work in the 1st, 2nd and 4th week are not true 1rm's and multiple reps are done so you should be able to maintain your thoracic. Louie Simmons is big on not maxing more than 3 weeks in a row on the same lift as your CNS will crash on that lift and you'll stall at some point.
So i would do Rhomboid, Middle and Lower Trap activation exercises before you lift to turn them on for when you lift, and increase their role in the lift, and then the same again using more of a strengthening approach after you lift, til you bring them up and can do a true 3rm with good form. Does that make sense?
The lumbar and cervical spine also directly affect the thoracic but this post is long enough. I hope that makes sense as this is a bit of a pet peeve of mine. Your only as strong as your weakest link. Isolate to integrate blah blah blah :))))