Third rep was the best where you got the back set and didn't jerk it off the ground. Loosen the belt. There's no reason it needs to be that tight until you're pulling 300kg for a 3rd attempt. It's going to make getting the back set consistently through the set difficult with it that tight right now.
Squeeze up the chest to set the back until you feel like your arms are going to rip out and then "peel" the bar off the ground.
Save up for a much better bar. I hate that bar.