На вашей фотографии у вас нет отчества, поэтому я не знаю, хотите ли вы этого. Но что вы можете сделать, если я вас правильно понимаю; Введите имя, фамилию и отчество display: inline-block, чтобы они располагались в одной строке.
.registrant-form__first-name__line,
.registrant-form__middle-name__line,
.registrant-form__last-name__line {
display: inline-block;
}
@import url('https://fonts.googleapis.com/css?family=Oswald&display=swap');
@import url('https://fonts.googleapis.com/css?family=Montserrat&display=swap');
a {
color: #23B7FB;
cursor: pointer;
text-decoration: none;
}
body {
font-family: 'Oswald-Bold';
color: #797979;
text-rendering: optimizeLegibility;
-webkit-font-smoothing: antialiased;
background-image: none;
}
#header {
background-color: #fff;
background-image: none;
border-top: 1px solid #ccc;
height: 0;
}
fieldset legend span {
width: 90px;
height: 55px;
color: #ff0031;
font-size: 37px;
font-family: Oswald-Bold;
font-weight: bold;
}
#main form ul li.registrant-form__name-title__line {
display: flex;
flex-direction: row;
}
input#registrant_first_name {
display: flex;
flex-direction: row;
}
ul.stacked .with.d {
display: flex;
flex-direction: column;
}
.registrant-form__first-name__line, .registrant-form__middle-name__line, .registrant-form__last-name__line {
display: inline-block;
}
<!DOCTYPE html>
<html>
<head>
<title>Page Title</title>
</head>
<body>
<fieldset>
<legend><span>Name</span><img mouseover="buttons/help_icon_over.png" alt="Help" class="tooltip" id="tooltip-registrant_name" title="Put your full name and in these boxes. Please do not use nicknames or initials. If this application is for a change of name you will be asked for your previous name in a later section. And don't forget to include your title (Mr., Mrs., Miss, Ms.)."
src="//s3.amazonaws.com/rocky-assets/assets/buttons/help_icon-bbee0add7434a7895a446d9bb29279210486f843afea4d4ae67a54f262979001.png" /></legend>
<ul class="stacked">
<li class="width_d registrant-form__name-title__line">
<h3><label for="registrant_name_title">Title</label><span class='required'>*<span class='required--text' style='display:none;'>(Required)</span></span>
</h3>
<div>
<select data-client-validation-required="Required" name="registrant[name_title]" id="registrant_name_title">
<option value=""></option>
<option value="Mr.">Mr.</option>
<option value="Mrs.">Mrs.</option>
<option value="Miss">Miss</option>
<option value="Ms.">Ms.</option>
</select>
</div>
<span class="error"></span>
</li>
<li class="width_b registrant-form__first-name__line">
<h3><label for="registrant_first_name">First Name</label><span class='required'>*<span class='required--text' style='display:none;'>(Required)</span></span>
</h3>
<div class=""><input data-client-validation-required="Required" type="text" name="registrant[first_name]" id="registrant_first_name" /></div>
<span class="error"></span>
</li>
<li class="width_c registrant-form__middle-name__line">
<h3><label for="registrant_middle_name">Middle</label></h3>
<div class=""><input type="text" name="registrant[middle_name]" id="registrant_middle_name" /></div>
<span class="error"></span>
</li>
<li class="width_b registrant-form__last-name__line">
<h3><label for="registrant_last_name">Last Name</label><span class='required'>*<span class='required--text' style='display:none;'>(Required)</span></span>
</h3>
<div class=""><input data-client-validation-required="Required" type="text" name="registrant[last_name]" id="registrant_last_name" /></div>
<span class="error"></span>
</li>
<li class="width_d registrant-form__name-suffix__line">
<h3><label for="registrant_name_suffix">Suffix</label></h3>
<div>
<select name="registrant[name_suffix]" id="registrant_name_suffix">
<option value="">(none)</option>
<option value="Jr.">Jr.</option>
<option value="Sr.">Sr.</option>
<option value="I">I</option>
<option value="II">II</option>
<option value="III">III</option>
<option value="IV">IV</option>
<option value="V">V</option>
<option value="VI">VI</option>
<option value="VII">VII</option>
</select>
</div>
</li>
</ul>
</fieldset>
</body>
</html>