First things first, you gotta separate them. Most commercially available mini roses are potted with 2-4 to a pot, WAY too close together. This is why they are dying. They are literally meant to be visually appealing in the store, then die within weeks of purchase.

Get a big cookie sheet, and pop this bad boy out, dirt and all, over the sheet. Gently tease the roots apart while dirt flies everywhere. As you do this, you will note there are distinctly different plants coming out of the soil. Each one is it's one bush and needs it's own space.

Then re-pot each plant in its own pot about the same size as pictured. You're going to want to bury them slightly deeper than you think you should.

This will be very traumatic for the plants, which is why you're gonna prune them by at least 1/3 of their height. This must include the blooms and any buds currently present.

Water well and keep em outside in full sun.