The “display:flex” style is a good way to make these elements in same line. No matter what kind of element in the div. Especially if the input class is form-control,other solutions like bootstrap, inline-block will not work well.
Example:
<div style="display:flex; flex-direction: row; justify-content: center; align-items: center">
<input type="button" value="Previous" id="btnPrevious"/>
<span id="spanStage">Stage 5</span>
<input type="button" value="Next" id="btnNext"/>
</div>
More detail about flexbox: https://css-tricks.com/snippets/css/a-guide-to-flexbox/
flex-direction: row, column
justify-content: flex-end, center, space-between, space-around
align-items: stretch, flex-start, flex-end, center