Bellevilles are pretty easy.
Stack them nested to increase spring rate. ((((( each spring adds to the spring rate. So 5 stacked would be spring rate*5. Doing this does not increase the travel of the springs
stack them staggered to increase spring travel, but maintain spring rate ()()()(). The spring rate does not change but the travel is multiplied by the number of springs
And then you can make combinations of these to get the spring rate and travel that you need. (())(()).
for the springs you have, even nesting two together will get you 3600lbs. You may want to get some lower rate ones and stack them to get closer to 3000lbs. Or hope you can turn the air pressure up more!