Trinity in Tempest

That Time I Got Reincarnated as a Slime: Trinity in Tempest (転生したらスライムだった件 異聞～魔国暮らしのトリニティ～) is a side story manga by Tae Tono.

Synopsis
The new spinoff manga to the hit fantasy isekai series, developed by original creator Fuse! Against the odds, the little slime Rimuru has established his magical kingdom for all monsters, called Tempest, and it’s thriving. But three visitors, Phos the fox girl, Stella the dragon girl, and Frey the winged girl, come to pay Tempest a visit, they’re stunned at just how quickly it’s developed. Don’t miss this new story in the world of Slime, just as the second season of the anime arrives!