If you don't mind using grid instead of flexbox, you can utilize the property justify-self to align the black box to the center with the help of position: absolute;. The absolute positioning here makes it so it's possible to align the black box toward the center without minding/affecting the other two boxes. See the snippet below:
.parent {
display: grid;
justify-content: flex-end;
grid-auto-flow: column;
background-color: lightgray;
position: relative;
}
.child {
width: 200px;
height: 200px;
background-color: gray;
border: solid black 1px;
}
.center {
background-color: black;
justify-self: center;
position: absolute;
}
@media screen and (max-width: 992px) {
.parent{
display: flex;
flex-wrap: wrap;
}
.center {
position: relative;
}
}
<div class="parent">
<div class="child center"> </div>
<div class="child"> </div>
<div class="child"> </div>
</div>
PS. I've added a @media query in case you want the 3 boxes to stack on a smaller space/screen.
Edit: Changed display on smaller screens and applied wrapping when necessary using flex-wrap.
More on justify-self and grid here and here respectively.