You need something like this: http://jsfiddle.net/Gajotres/a5USz/
HTML:
<div data-role="footer" data-position="fixed">
<div class="button-holder">
<input type="button" value="Button"/>
</div>
<div class="button-holder">
<input type="button" value="submit"/>
</div>
<div class="button-holder">
<input type="button" value="cancel"/>
</div>
</div>
CSS:
.button-holder {
text-align:center;
width:100%;
}
.button-holder .ui-btn {
width: 100% !important;
}
Add margin: 0 if you want to make them side by side, like this:
.button-holder .ui-btn {
width: 100% !important;
margin: 0 !important;
}