Mahou Shoujo Ikusei Keikaku: breakdown

An experiment gone wrong leads to the death of a famous mage, and the passing of this illustrious individual sends the Magical Kingdom into mourning. Then, a few months later, the deceased mage’s relations receive letters from his proxy regarding the inheritance of his estate, inviting them to a tiny uninhabited island the mage had used as a getaway cottage/research lab. When Mana gets one of these letters, she’s skeptical, but after careful consideration, she contacts two magical-girl friends to accompany her there...

(Source: Yen Press)