DevExpress create every time a element in the DOM, therefore sometimes couldn't be straightforward to customize the page. Anyway you can:
1) Use ASPxThemeBuilder to modify your DevExpress theme;
2) CSS:
<div style="float: left;"> Html.DevExpress().TextBox.. </div>
<div style="float: left;"> Html.DevExpress().TextBox.. </div>
<div style="float: left;"> Html.DevExpress().TextBox.. </div>